What do you suppose they hoped to convey to your brother with that statement?
It's quite possible that their experience with black guests was negative. But by telling your brother of those negative experiences (which he had no need to know), weren't they, at least in some sense, effectively differentiating themselves from another, negative category of blacks?
They told him this after he was there for a while and got to know them, and he said it came up in normal conversation.
I suppose they could have been trying to differentiate themselves, or, they could have been attempting to bond with him over their (perceived on my brother's part) mutual dislike of lower class black people.
But given the situation, I think it's more likely that they had no ulterior motive. They had a few bad experiences with black guests, and decided to use race as a heuristic for determining suitability as a guest.
This is how nearly all racism starts, it's unfair and we should strive to eliminate it, but people are always going to make generalizations.
It's quite possible that their experience with black guests was negative. But by telling your brother of those negative experiences (which he had no need to know), weren't they, at least in some sense, effectively differentiating themselves from another, negative category of blacks?