Somewhere I remember reading about OLD brass bodied heavy pro film cameras getting painted black while cheaper metal 35mm bodies were lighter metal with I think a chrome finish.-- This could be where the choice of "black" in general as a pro color comes from or it could be that "bling" is counterproductive (photojournalism in war zones, wildlife photography) or unnecessary (few people waiting for the D900 because of the button...). The D7000 has partial alloy body so it's possible they denote metal bodies with black button and non-metal bodies with the plastic so you don't have to slam them hard against the sales counter to figure out which body you have. P7000 I think has black buttons...

Why do consumer DSLRs from Nikon have the chrome button while the D7000 up have the black button?