On the pages of many modern web services (including stackexchange webpages), there is a black header. What is the psychological effect of this? Is there any significance of it being black? Is there an optimal width for this header?
I think it will largely depend on the context of which it is apart of. StackExchange and Google (although Google has changed recently) both use a ~35px-height top header, but they also both have other web applications of interest. As well, they have profiles that encompass many of their web applications. I believe this is their method for keeping those applications/sites available throughout multiple applications. I'm fairly certain the black is to differentiate it from the actual site/app, but that may be only one of the reasons.
As for optimal width I would say it should span the full page (100%).