History of "white keys" and "black keys" as idiom

Question

When explaining music theory in layman's terms, it's common to refer to "white keys" and "black keys" and take for granted that everyone knows what a piano looks like.

But keyboards didn't always look like that. Early keyboards often had black natural keys and white accidentals, or black and brown, or something else entirely. While exceptions still exist today, at some point the current look became sufficiently standard to serve as a reference point.

What's the earliest known reference to "white keys" or "black keys" where the audience was meant to assume the modern arrangement without further specification?

Answer 1

A fairly cursory search in Google Books finds the usage attested in 1731 in The Modern Musick-Master; or the Universal Musician ... containing, i. An Introduction to Singing ... ii. Directions for playing on the Flute ... iii. The Newest Method for Learners on the German Flute ... iv. Instructions upon the Hautboy ... v. The Art of Playing on the Violin ... vi. The Harpsichord Illustrated & Improv'd. In which is included a ... Collection of Airs, and Lessons ... extracted from the Works of Mr. Handel, Bononcini, Albinoni, and other Eminent Masters. With a Brief History of Musick ... To which is added, a Musical Dictionary ... Curiously Adorn'd with Cuts ... Finely Engrav'd on above 320 Plates:

Observe in the foregoing Example of the Gammut that there are twenty-nine white Keys [which is the number contain'd in many Harpsichords except in those made here of late : to which they add both above and below, sometimes to the number of thirty-seven] There are also twenty black Keys somewhat shorter than the others which are placed between them and serve for flats ♭ ♭ or sharps ♯ ♯ to the white Keys, for Example the short Key that is between G & A serves for both G♯ and A♭, the short key between A & B serves also for A♯ and B♭ &c for the rest.