Why is the MacBook keyboard black instead of grey?

There are a couple reasons. The first is the longevity of the keycap legibility. The second regards an international standard for keycap contrast. You're probably scratching your heads asking, "Is there really an international standard regarding the ability to read a keycap?" Indeed there is. When SJ returned to Apple, one of his first decisions was to change the keycap color of a portable computer from black to "cola/bronze". Hysteria ensued - don't get me started.

When you are a two finger typist like me, the key color becomes important. Having MBPs with both types of keys, I find that the black ones are much more easier and practical to deal with. I may have to look at the keys but never the trackpad. It is too intuitive to have to do otherwise.

I do have to admit that from an esthetic perspective, the silver key MBPs look just a little nicer.

Ciao.

A silver keycap with a white letter does not provide sufficent contrast as legislated. A silver keycap and a sufifciently contrasting letter color would be very difficult to illuminate. The current ID solution provides sufficient contrast, and the ability to illuminate the letter.

"Do you have any sources that describe the standard?"

I believe the inital legislation was passed by the EU, and it was part of ergo standards, including usability by the physically limited.