One way to name a black key is to use a flat symbol (b)…

“Flat” means the very next note down (to the left).

For example: Bb, a black key, is the very next note down from B…

Here are all the black keys named using flats…

By the way, a flat does not have to be a black key! Two white keys can also be named using a flat. Notice that Cb, the very next note down from C, is a white key and Fb, the very next note down from F, is also a white key…

Putting it all together, here are all seven possible flats…

learn more… Names of the Piano Keys: Double Sharps & Double Flats