Perhaps Gainsborough's most famous work, it is thought to be a portrait of Jonathan Buttle (1752–1805), the son of a wealthy hardware merchant, although this has never been proven. It is a historical costume study as well as a portrait: the youth in his seventeenth-century apparel is regarded as Gainsborough's homage to Anthony van Dyck, and in particular is very close to Van Dyck's portrait of Charles II as a boy.
Gainsborough had already drawn something on the canvas before beginning The Blue Boy, which he painted over. The painting is about life-size, measuring 48 inches (1,200 mm) wide by 70 inches (1,800 mm) tall. Gainsborough painted the portrait in response to the advice of his rival Sir Joshua Reynolds, who had written:
It ought, in my opinion, to be indispensably observed, that the masses of light in a picture be always of a warm, mellow colour, yellow, red, or a yellowish white, and that the blue, the grey, or the green colours be kept almost entirely out of these masses, and be used only to support or set off these warm colours; and for this purpose, a small proportion of cold colour will be sufficient. Let this conduct be reversed; let the light be cold, and the surrounding colour warm, as we often see in the works of the Roman and Florentine painters, and it will be out of the power of art, even in the hands of Rubens and Titian, to make a picture splendid and harmonious.