After we started seeing the black and sleek YouTube player, Google also seems to be testing a black header, as you can see in the screenshot here, for some people right now.
Initially, some people were seeing this new, slightly overhauled look when not logged in. I have seen this before too. But for the very first time, today, I opened the web browser, already logged into my Google Account and saw the black header.
I have no idea what's with this black overhaul of all Google websites. Why are they turning to the 'dark side'? I think the new YouTube player looks amazing. But I don't think the black header on Google's search engine pages matches the overall look very well. We'll have to see what Google is planning for the future and what other changes are going to come, if any.