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.
The day after the trial, the missionary society thinks their paid "darky's" aren't being very good Christians. They are "sulking" around. One of the ladies even called them hypocrites stating that they weren't being very good witnesses for the Lord.
This is ironic because the people saying that are the ones showing insensitivity and not being very good Christians.
They also talk about Helen Robinson negatively. At one point they are cutting on a woman and Scout can tell. For a while she thinks its Mayella because the description is so terrible, but then comes to find out its Helen.