Been reading about Google's new dark navbar over at ReadWrite. It's funny that ReadWrite, as well as StackExchange, has chosen a dark navbar as well.
Why this trend to choose dark navbars?
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I'd say it's a fairly common design choice due to the constraints of the media - rather than a trend.
What do you want a navigation bar to do:
1) You want people to find it easily when they need to navigate
2) You want it to get out of the way visually when people are focussing on the content.
3) In the case of sites like Google, where you have a common bar across multiple services, you want something that's not going to clash visually with different brands / colour schemes / etc.
Since most people present content dark-on-light then (1) and (2) will probably drive you to a navigation bar that's more likely to be light-on-dark so it's obvious what's content and what's nav bar.
(3) is probably going to drive you to neutral colours / greys that avoid colour scheme clashes.
Since most browser chrome is grey, you'll probably want to go for a pretty dark background to clearly separate your navigation from browser navigation / bookmark bars / etc due to (1).
So the design constraints drive you towards a dark grey / black bar.
Nav bars need to stand out and be visually distinct. Many if not most sites have a light background with a dark foreground (text); to give a good contrast, a dark navbar on a lighter site is a no-brainer.
Look at the Read Write page and try to imagine a light nav bar; it'd either be clashy (few light colors would work there) or hard to see (white/gray). In addition, black goes with (almost) anything, so it's a good safe choice. Adding colors is always a risk in a design, but adding white and black can generally be done in most color schemes without messing them up.