Google hasn't been doing much to its search site recently, the biggest change was the switch to the horizontal toolbar which replaces the left sidebar.Other than that, the site has stayed the same for quite a while, a somewhat unusually long while knowing Google.
But it's not going to be for long, Google is always running design experiments and it seems that it's now focusing on the top navbar again, this after a failed attempt to get rid of it altogether last year.
Several users are reporting that the black navbar has now gone gray, though there don't seem to be any other changes to it.
The black navbar has been around for a few years and survived attempts to replace it with a unified Google menu.
When it was getting ready to roll out Google+, there were some white variants of the navbar in testing, but Google settled on the black. It may be testing a new color now or it may even be rolling it out to all users, we'll find out soon enough.