Worth noting that a few weeks ago we had a discussion in Supporters Google about a test involving changing the Google ad markers
from orange to green [webmasterworld.com
...] (thread requires Supporters access)
The OP's speculation in the Supporters discussion was that changing the markers to green would make it less obvious that the ads were ads. But if the intention were simply to increase ad clicks, I suggested, green would be less likely to attract the eye, another factor in how many clicks ads receive. Considerations about the colors of rating stars, in both ads and organic, might also eventually enter into it, as they're also yellow orange and attract the eye.
My guess is that black is one more variable in what might become a fairly complex set of tests involving many variables over time.
Personally, I think that black and green combined, as in the serps shown in the Register, is one of the more depressing color combinations imaginable.