Plenty of mechanical keyboards are black, especially the gaming ones. Razer Blackwidow is black:
http://store.razerzone.com/store/razerusa/en_US/pd/productID.211324200Deck is black:
http://www.deckkeyboards.com/Steelseries makes them black too:
http://steelseries.com/products/keyboards/steelseries-7gAnd many, many others not designated as gaming pieces (ABS M1, some Cherries, TVS Gold, Solidtek ASK6600, and obviously the Das Keyboard units, which are actually piano black). I suppose most mechanical keyboards that aren't very old are black. The white colour dates back to when most computer cases were white.
Also, Unicomp makes Model M keyboards (the "buckling spring", which is not strictly speaking a mechanical keyboard as much as it tends to be seen as the definition of one) in the black colour.
I'd suggest trying in the shop, followed by starting with used pieces, to see what kind of feel you like the best. For example, most people like Cherry browns and I don't.
From your description, though, I'd probably go with this if I had the money (Razer Blackwidow ultimate):
http://store.razerzone.com/store/razerusa/en_US/pd/productID.211651300But you really should give the old buckling spring IBM keyboard a go. In the Mods section of this forum there's plenty of threads on painting it black in a professional manner but unprofessional works great too. Heck, my sister has a keyboard painted with nail varnish with hand-drawn flowers on it. Myself, I'm inclined towards gold metallic and gluing fake stones to it, just for the kicks...