I like contrasts. Some of my boards are white, some beige, some black. My most recent acquisitions include the HHKB Pro 2, RF 87u, and CM Novatouch. All were purchased in black with black keycaps, but I replaced the alphanumeric keycaps on the HHKB and RF with white keycaps (black legends) from a HHKB Pro 2. The Novatouch currently has blue alphanumerics (black legends) and gray/beige mods (black legends).
Edit: I like the black RF 87u with blue alphas and black mods so much, I am considering the same color scheme for all my black keyboards. Previously, my RF was all black, but I prefer legible legends on the alpha keys. However, the contrast was too stark with white keys and black legends on a black keyboard. The sky blue keys dials down the contrast to a level that is soothing and functional.
I've also tried expanding the blue keys to the F-key row and the Navigation Island, but this looked too busy. However, the RF 87u looks good with blue arrow keys in addition to blue alphas, with everything else in black.
My vintage IBM boards are a different story. They look fine in their original beige and off-white, although some look great with the case repainted in certain dark colors (but not necessarily black).