Its certainly the case that black actors do feel restricted in the sort of roles that they are offered, and thats particularly true of the UK, as these articles make clear - &

There is a reason why you see so many Black and Asian British actors in US productions - they get roles in The Wire, Homeland, Lost, The Good Wife, etc. Admittedly, white British actors are also appearing in the same shows, but black actors simply have more chances of being cast in the sort of meaty roles that are thin on the ground here. Things are getting better, with Lennie James excellent in The Line of Duty, Lennie Henry has had great reviews for his theatre work, and Idris Elba in Luther is starting to make a lead black actor something that is normal, but we still have some way to go. A quick google about the role of Guinevere in Merlin shows that people are still have too much time on their hands...

Female actors tend to have a more difficult time than male actors when getting cast - as Agutterfan put it:
To be honest, there are very few films made these days with decent roles for women, so the competition is fierce.
Being a black women means your a subset of that group. However, 25 years ago, a black leading man in a mainstream Hollywood film would have been problematic, now you have a number of A list stars. Increasingly, Hollywood will move to be 'post-racial' - if your a big name and you sell tickets, race will be less of an issue (the demographic changes in the US will also play a part).

As for black women playing leading roles, this article about Scandal (which I'm watching while typing this) shows that such a thing can be done, and deliver. I'm just waiting for it to happen in Britain.