When Atticus says that Mr. Cunningham is basically a good man, even though he has blind spots like everyone else, he isn't speaking literally. He means that Mr. Cunningham is a kind man overall, but, just like everyone, is imperfect, often misjudging situations out of ignorance. For example, in this time (though is not an excuse whatsoever) there were many lynchings and extreme racism. At night, Mr. Cunningham approached the jailhouse where Atticus kept watch. Mr. Cunningham was part of a racist mob coming to attack Tom, while Atticus stayed nearby for the possibility of exactly that happening. Of course, Scout was watching, and when she saw the men, she leaped out of hiding. The men told Atticus he had 15 seconds to get her to leave, but Scout being how she was, kept talking. She treated them will kindness in spite of their threatening demeanor, even asking if Mr. Cunningham could tell his son "hey" for her. This brought Cunningham to his sense, realizing what he was about to do. Scout's ability to see humanity in the inhuman nature of the men standing before her, a racist mob, caused Mr. Cunningham to retreat. Atticus, like Scout, saw humanity in the men, hence his comment referring to Mr. Cunningham's participation in a racist mob as a "blind spot." Atticus stood to protect a man from racists, serving as a peacemaker rather than taking one side or another. The men at the jail looked at people more as good or bad, but Atticus saw them as imperfect beings, with a little of both in their nature. Not to mention, Atticus shows his position on the situation with peaceful, proactive choices, whereas the men make aggressive, reactive choices to display their views. Hope this helps!
