One of the acceptations of the French noun cordon is: a ribbon, usually worn scarf-wise, as part of the insignia of a knightly order.

The original meaning of the French term cordon bleu (literally blue ribbon) is: the sky-blue ribbon worn by the Knights-grand-cross of the French order of the Holy Ghost, the highest order of chivalry under the Bourbon kings.

In this posthumous portrait by E. Trochsler, François Gaston de Lévis (1720-87) is wearing the blue ribbon of the French order of the Holy Ghost—image Wikimedia Commons:

The term cordon bleu was extended to other first-class distinctions. For example, in Mémoires Anecdotes, the French author, translator and member of the Académie française Jean Regnault de Segrais (1624-1701) wrote:

Je n’ai pas trouvé dans le Ménagiana [note 1] ce que j’avois dit à Monsieur Ménage, & dont il étoit convenu, que l’Academie Françoise étoit le Cordon-bleu des beaux Esprits ; il le disoit souvent comme venant de moi.
     translation:
I have not found in the Ménagiana [note 1] what I had said to Mister Ménage, and which was agreed on, that the Académie française was the great minds’ cordon bleu; he often said it as coming from me.
—source: Œuvres diverses de Mr. de Segrais. Tome I (Amsterdam: François Changuion, 1723)

The term cordon bleu was also applied to the wearers of the above-mentioned insignia, and by extension to other persons of distinction.

In the sense of a first-class cook, cordon bleu is first recorded in Almanach des gourmands, servant de guide dans les moyens de faire excellente chère (The gourmands’ almanac, serving as a guide to the means of making excellent cheer – Paris: Maradan, 1804), by Alexandre Balthazar Laurent Grimod de La Reynière (1758-1837) and Charles-Pierre Coste d’Arnobat (1732-1808?):

Les érudits en bonne-chère se souviennent que le premier restaurateur de Paris, nommé Champ-d’oiseau (1), établi rue des Poulies, ne date que de 1770. […]
(1) Ce M. Champ-d’oiseau, premier restaurateur de Paris, est aujourd’hui dans la misère, et il seroit digne de ses opulens successeurs de lui assurer, en se cotisant, une petite pension viagère ; à-peu-près comme les comédiens donnent souvent des représentations au profit de leurs camarades infortunés. Cet homme ayant en quelque sorte imaginé et fondé l’état de restaurateur, est la cause première de la fortune des Méot, des Robert, des Beauviliers, des Naudet, des Very, etc. ; et un léger sacrifice de la part de ces cordons bleus de l’ordre assureroit à l’infortuné Champ-d’oiseau une existence au-dessus du besoin.
     translation:
The persons erudite in good cheer remember that the first restaurateur in Paris, named Champ-d’oiseau (1), established rue des Poulies, dates only from 1770. […]
(1) This Mr. Champ-d’oiseau, first restaurateur in Paris, is today in poverty, and it would be worthy of his opulent successors to assure him, by clubbing together, of a small life annuity; rather like the comedians often give performances for the benefit of their unfortunate comrades. This man having as it were imagined and founded the trade of restaurateur, is the primary cause of the fortunes of the Méots, the Roberts, the Beauviliers, the Naudets, the Verys, etc.; and a slight sacrifice from those cordons bleus of the order would assure the unfortunate Champ-d’oiseau of an existence above want.

The French playwright, librettist and member of the Académie française Étienne de Jouy (1764-1846) also used the term cordon bleu in the sense of a first-class cook in the fifth volume of L’hermite de la Chaussée-d’Antin, ou observations sur les mœurs et les usages parisiens au commencement du XIXᵉ siècle (The Chaussée d’Antin hermit, or observations on the Parisian mores and usages at the beginning of the 19^th century – Paris: Pillet, 1814):

J’ai remarqué ces cuisinières de bonnes maisons, connues dans la livrée sous le nom de cordons-bleus, et qui, trop paresseuses pour aller aux Halles [note 2], dédaignant les marchés bourgeois du faubourg Saint-Germain, vont faire leurs emplètes chez les marchands de comestibles du Palais-Royal, au risque de payer un tiers de plus des provisions qu’elles font payer le double à leurs maîtres.
     translation:
I have noticed those female cooks of wealthy households, known in the domestic service under the name of cordons-bleus, and, who, too lazy to go to les Halles [note 2], disregarding the middle-class markets of the faubourg Saint-Germain, make their purchases from the vendors of eatables of the Palais-Royal, at the risk of paying an extra third for provisions for which they will charge their masters twice the price.

In the sense of a first-class cook, cordon bleu is first recorded in English in the second volume of Reminiscences of Michael Kelly, of the King’s Theatre, and Theatre Royal Drury Lane (London: Henry Colburn, 1826), by the Irish singer and composer Michael Kelly (1762-1826)—the following is about the Duke of Queensberry:

His chief French cook, whom he denominated his officier de bouche, was a great artist, a real cordon bleu, who ought to have had, like Cardinal Wolsey’s master-cook, a crimson velvet dress, with a collar and a gold chain.

NOTES

1 Gilles Ménage (1613-92) was a French grammarian, historian and author. After his death, his friends published Ménagiana (Paris: Florentin et Pierre Delaulne, 1693), containing “the witticisms, the judicious and moral thoughts, and the curious observations, collected from the mouth of the late Mr. Ménage” (“les bons mots, les pensées judicieuses & morales, & les observations curieuses, recueillies de la bouche de feu M. Ménage”).

2 Les Halles was until 1971 the central fresh-food market of Paris.