What are some of the opinions among the missionary circle concerning blacks?

I agree with the first answer as far as it goes.  I would also point out a couple of other things.

First of all, the main idea that the women have about blacks (in my opinion) is that the blacks should know their place and be grateful for what they white people are giving them.  The women talk about how people who think they're helping blacks (Atticus) are really just giving them ideas about getting uppity.

Second, as to your second question, Aunt Alexandra is thanking Miss Maudie for scolding Mrs. Merriwether.  Aunt Alexandra is kind of stuck up herself, but she does not like how badly Mrs. Merriwether is talking about Sophy.  I think it's because she thinks high class people need to be more gracious toward the people beneath them.