The phrase a blessing in disguise designates an apparent misfortune that works to the eventual good of the recipient.

This hackneyed phrase occurred, for example, in Sarah’s Stars, by Sarah Delamere Hurding, published in the Irish Independent (Dublin, County Dublin, Ireland) of Tuesday 1^st December 2009 [Vol. 118, No. 286, page 28, column 7]:

Aquarius
21 January to 19 February
Leave what is done with in the past and do not call it back in with negative thinking. Routine is a nuisance; but you gotta do what you gotta do! A clash of wills turns out to be a blessing in disguise. So be philosophical and do not panic. Expect this to be a whirlwind stretch. You may not even have time to think.

The phrase seems to have first been used in the plural form blessings in disguise in the first half of the 18^th century; the earliest occurrences that I have found are as follows, in chronological order:

1-: From Book III of A Poem on the Last Day (Oxford: Printed at the Theatre for Edward Whistler, 1713), by Edward Young, Fellow of All-Souls College, Oxon [page 58]:

Us’d with Art, and rightly understood,
All Dispensations from Above are Good;
And tho’ with frightfull Aspect they Surprize;
Most Ills are only Blessings in Disguise.

2-: From Sermon VII. Patience under Afflictions an Ornament to Religion, in Several Sermons, chiefly upon Practical Subjects, many of which were preached before Her Present Majesty, when Princess of Wales (London: Printed for R. Gosling and E. Symon, 1730), by the Church of England clergyman and poet Nicholas Brady (1659-1726) [page 140]:

We [are] apt to call in question God’s Goodness, when we find our selves encompassed by manifold Calamities, but if we reflect that they are designed as Medicines, to melt down a hard Heart, or heal a wounded Conscience; if we consider, that these light Afflictions, which are but for a Moment, are to rescue us from endless Ages of insupportable Torments, and to work for us a more exceeding and eternal weight of Glory, we cannot be so far biassed by our present Sense of them, as not chearfully to submit to them, and adore that Goodness, which draweth Consequences from them so much to our Advantage. We are told, that whom the Lord loveth those he chasteneth, and he even chasteneth whilst he loveth them; his very Judgments are but Blessings in disguise; and we may so sanctify them by a Christian Resignation, as to make them bring forth the peaceable Fruits of Righteousness, to such as are duly exercised thereby.

3-: From Reflections on a Flower-Garden. In a Letter to a Lady (London: Printed for J. and J. Rivington and J. Leaks, 1746), by the Church of England clergyman and author James Hervey (1714-1758) [page 76]:

Since all the downward Tracts of Time
God’s watchful Eye surveys;
O! who so wise to chuse our Lot,
And regulate our Ways?

Since none can doubt his equal Love,
Unmeasurably kind;
To his unerring, gracious Will,
Be ev’ry Wish resign’d.

Good when He gives, supremely Good;
Nor less, when He denies;
Ev’n Crosses from his sov’reign Hand
Are Blessings in Disguise.