Can't speak for the other two, but Worcester is pretty good
It's big, pretty old (has some very old buildings) it has a good social life as oxford colleges go and it offers accommodation for 3 years
Sadly, Oxford is not hugely ethnically diverse as universities go (although it is improving this diversity). You shouldn't worry about being the only black student at any of the colleges you mention, they are all reasonably large colleges and so each will have at least some black students in all likelihood.
In terms of any one college being a 'good fit' for ethnic minorities I would say that pretty much every college will be similar. My friends at Worcester from ethnic minorities have had no trouble (to my knowledge) fitting in. Indeed one of them was elected JCR president last year.
so, I think you'll get a similar experience at most colleges but Worcester is pretty good, I've had a great time there (should point out I'm not from an ethnic minority) and hopefully you will too if you choose to apply there
EDIT: seems that Brasenose admissions roughly agrees with me. The most important point is that college to college the experience will be broadly similar