Think about it. In the key of C, all keys are white, so its can be a little more confusing because one can lose their place as to what notes they are pressing. I've tried showing some people some things on they keyboard in C and I'll like play the a C major and then play a F major...and tell them to play it...just after watching me, they play something totally different.
With black keys, you have, sort of..like markers, to remind one of where the notes are....if you get what I mean.