It's a language thing. Racists (and we're going a long way back here, slave owners etc) don't care if you're African, African American, or anything inbetween so it's all "black", and that creates a group in itself who can self identify as part of it and create oppositional discourses.
That's my take anyway.
They normally don't care if you're Asian or Eastern European (in this country) either...
Why isn't there a major Asian pride movement in the US? They too (largely Chinese) were used as slave labour and treat absolutely horrendously. Whilst it wasn't on the same scale that makes no difference on a personal level to those that suffered. It still continues to this day.
The UK has a very different and much more positive history with race. Its not been perfect and still isn't but we're pretty close. So again...what have these individuals to be proud about?