Just from personal experience (really long blond hair, though mine is closer to brown than yours) black doesn't work too well, the only colors I can pull off are non-natural colors, blue and green worked well when I had a mohawk. If you're going to try all black, make sure you get the temporary stuff first, even if it's the really cheap stuff, just to see if the color works, then you can get it off and either switch colors or go for the permanent one.
Even if you end up not changing it at all, I find that I stick out more as mine is now, not a whole lot of people can pull off the long blond hair goth look
