> But it, and the piano black keys with it, are definitely not optimal. It makes playing in C easy, and playing in anything else hard.

I don't think this is true—I was under the impression that, at least on piano, keys other than C major are sometimes chosen specifically to make things easier on the fingers.

Easier on the fingers - maybe (I don't know what you reference), but not easier on the mind: Instead of learning each chord once, you have to learn it 12 times (for each key).

See http://terpstrakeyboard.com/about/