I am facing this issue after installing VS 2012. By some reason XAML editor in VS2012 doesn't show any syntax highlighting. It is just showing black text and even comments are not coming in green.
Its very difficult to understand xaml without syntax highlighting. If you are also facing this issue then below command might help you to fix this issue.
go to run -> type -> devenv /resetuserdata
then press enter.
It might fixed the XAML highlighting issue.