I think it probably is tougher in the UK for a black sister to find a marriage mate, just because there are less of them, that's all. If she didn't necessarily care whether her husband was black or not it would make it slightly easier for her. I've no doubt that the racism of individual brothers/families could also make things tougher. But I don't think the society is racist for using a black sister, I think they are trying to portray quite the opposite.
NB Cofty you cannot be serious about WT having a "paternal" attitude to women. Maybe individual brothers/elders etc may have this attitude, but the society's attitude to women is and always has been toxic.