The noun copybook denotes an exercise book with samples of scripts, in which children practised their writing.

—Cf. also origin of criss-cross and origin of to do the i’s and cross the t’s.

The phrase a blot on one’s copybook designates a fault, misdemeanour or gaffe which blemishes one’s reputation.

The phrase to blot one’s copybook means to commit a fault, misdemeanour or gaffe which blemishes one’s reputation.

The phrase to blot one’s copybook occurs, for example, in the following from a letter by one Linda Evans, published in The Independent (London, England) of Monday 6^th December 2021:

How much longer Boris Johnson will be allowed to remain prime minister is debatable. There are only so many times one can blot one’s copybook and, by any standards, Johnson’s book is well blotted.

—Cf. also the phrase a blot on the landscape and the adjective blotto.

The earliest occurrences that I have found of the phrases a blot on one’s copybook and to blot one’s copybook are as follows, in chronological order:

1-: From the account of the Crewe Volunteer Fire Brigade contest that took place in Crewe on Monday 14^th April 1879, published in The Crewe Guardian (Crewe, Cheshire, England) of Saturday 19^th April 1879—the contest judge was Colonel Sir Charles Frith, Justice of the Peace, and, because some of the contestants disputed his decisions, the following was said during the banquet which followed the contest:

Dr. Atkinson said he was sure the next toast, that of “The Judge,” would meet with the heartiest reception. He […] did not think the competition would have been brought to so successful an issue if it had not been for the high and very prominent position which the judge who had come among them that evening had occupied. Sir Charles had just told them that judges had sometimes unpleasant and disagreeable duties to perform, and there was a saying that people who had disagreeable duties to perform sometimes performed them in a disagreeable manner; but in the whole of that company he was sure there was not one who could say Sir Charles Frith had not performed his duties in the most genial and agreeable manner possible. (Applause.) This was the first time Sir Charles had been in Crewe, and he hoped it would not be the last. (Applause.)
Sir Charles Frith, in response, thanked Alderman Atkinson for the very kind manner in which he had proposed the toast. He admitted that the circumstances referred to were rather delicate and trying to a judge, and he regretted that they should have anything like a blot on their copybook when they were having a little festive gathering.

2-: From The Macclesfield Chronicle and Cheshire County News (Macclesfield, Cheshire, England) of Friday 16^th May 1879:

“A BLOT ON THE COPYBOOK.”

“A blot on the copybook indicates carelessness,” said my grandmother, peering through her well-worn spectacles at my black effusions on the paper before me. “Look at it,” she said. Poor old granny! Many years have passed over my aching head since her thin gray locks were laid in the turfy bed of Stanton’s old green hillocks, but the lesson lingers fresh as the springing grass that tops her silent sod. Poor granny! What would she say could she but view my copybook of life, blackened with the fumes from the nether world, gnarled by the world’s hot warping fires, blasted by the mildew of false friends, or shrivelled with the lightning’s two-edged sword, and revealing forgotten deeds from whose recesses the skeletons of bygone years stalk forth with grave, weird scrolls of duties left undone; of madhouse freaks performed in manhood’s manless moods, mingled with sights that float on the fleecy wings of memory—forgotten as conceived—glimpses of the unseen foreshadowing the realities of the future in the womb of the unborn. Unbidden as the Eastern wind they come; unrequested as a midnight dream they emerge from the land of existence into the misty gleams of a maddened memory, leaving only the blots of the past high up on the ledger lines of life. The past! Let it bury its dead. Why should its stealthy form revive to shame the glowing now? “Eat, drink, and be merry, for to-morrow——” But why think of to-morrow? The present moment is our own, the next we never saw.”
“To-morrow you will live,” you always cry;
In what far country does to-morrow lie,
That ’tis so mighty long ere it arrive?
Beyond the Indus does this morrow thrive.
’Tis so far-fetched, this morrow, that I fear
’Twill be both very old, and very dear.
To-morrow I will live, the fool doth say,
To-day’s too late: the wise liv’d yesterday.
But where in the hoary registry of time shall we find the pass-word to the caves of sweet oblivion? Where lies the fuller’s earth to white the sin-stained pages—the red-lipped lines that speak of blood and revelling? Where shall Nature’s swaddling-clothes be found to dress again the soul in all her pristine innocence? In creeds or codes of law? In schools or class of —“isms?” “Not there, not there, my child.” No, not in the well-written copy of a virtuous life, nor in the thousand and one acts of kindness, nor in the life spent for humanity does conscience find her rest, but in the unblotted pages of the present, enamelled by the hope borne on the morning air from the land where the Sun of Righteousness ariseth with healing in his beams. Oh, if our copybooks contained no blots but those of carelessness, then would the arrows that emerge from the white-hilled walls of heaven pierce with less than mortal effects. Then might man whose less stained leaves stand up to blot his nabours. Till then, “Conscience ‘will’ make cowards of us all,” throughout all generations, and grow with the growth of the germs that spring in Spirit-land, and mingle with the waves that bear the mariner over the sea to the ocean yet to come.
F. RAWLINGS.
Reddish, May 7^th, 1879.

3-: From The Macclesfield Chronicle and Cheshire County News (Macclesfield, Cheshire, England) of Friday 6^th June 1879:

Biblical Cosmography.—This is a pamphlet by John Hampden, aiming at confuting the Newtonian theory, and proving that the world is flat. The pamphlet will be found highly diverting by all classes of our readers. Some fifteen pages are occupied by a contradiction of the details of “current scientific theories” by the simple process of assertion, and the manner in which the verification of “current theories” is persistently overlooked, is something which in audacious impertinence verges on the sublime. It is, we know, the popular opinion that Mr. Hampden is in earnest in his expressions of opinion, but for our part we beg leave to doubt the fact. In some parts of his pamphlet we think we catch glimpses of an irony which suggests the writer is cynically amusing himself at the expense of his species, and trying to what lengths their folly can imposed upon. Be his purpose what it may, however, the diverting character of his little work stands unaffected. Considered as a travesty on the loose style of argument and reasoning current in our day, it is superb. We wish we could say that amusement was the only result of a perusal of a work of this kind, but, alas! there is a deep tragic irony in the very possibility of such a travesty. We call to mind sadly the number of cases in which the prevalence of views as gross as those advocated by Mr Hampden, retard the march of human progress, in which truths affecting social welfare not as obvious as those of astronomy are scouted through popular inability to group their cogency. This is a vein of thought not exactly in keeping with the first appearance of the book, but in view of actual circumstances it is not without its utility. If to absence of intellectual development are attributable many of the evils of the age, then clearly whatever exhibits this mental feature in its full deformity will have a commensurate value in stimulating and quickening a spirit of inquiry, and removing the “blot on the copy-book” referred to. Purposely or inadvertently, Mr. Hampden has given us a satire on bad argument, and in so doing he merits the thanks of those—and their number, if small, is increasing—to whom bad argument is “chiefest among things unclean.”

4-: From A Spinner’s Story, by J. T. Clegg, published in Ben Brierley’s Journal of Literature, Science & Art (Manchester, Lancashire, England) of Saturday 4^th December 1880—reprinted from Ab-o’th’-Yate’s Christmas Annual:

My mother coome in, an’ I towd her ’at I were determined to turn o’er a new leeafe at last, and be careful about blotting my copy-book again, an’ th’ owd gell put her apron o’er her face for a minute, an’ then geet up, lookin as cheerful as a spring mornin. There were no moore fuddlin for me after that. I began to howd my yed up once moore, an’ gave o’er bein freetent about lookin folk i’th’ face.
     in standard English:
My mother came in, and I told her that I was determined to turn over a new leaf at last, and be careful about blotting my copy-book again, and the old girl put her apron over her face for a minute, and then got up, looking as cheerful as a spring morning. There was no more fuddling for me after that. I began to hold my head up once more, and gave over being frightened about looking folk in the face.