It's possible you're thinking of a different artist, or you might be remembering a painting of his where blue is a prominent color. Some of his famous paintings include:
* "Portrait of Mrs. Siddons as the Tragic Muse" (1784): The background in this painting is a deep blue, representing the dramatic setting.
* "Portrait of Samuel Johnson" (1756): While not a dominant color, blue is used subtly in Johnson's clothing, adding depth to the portrait.
* "The Age of Innocence" (1788): The blue dress worn by the young girl in this painting is a defining element.
If you can provide more information about the painting you're thinking of, I might be able to help you identify it.