I would like to ask native speakers of English of the questions. The following story, which happened between a mother Jane and her daughter Mary, is a quote from a textbook.
As Mary stood there, a piece of paper fluttered out from between the pages of the human biology textbook Jane was waving angrily at her. Jane picked it up. It was a watercolor of a bird in flight. The art was simple and self-taught. But there was a power, a wild grace in the open wings that Jane fad failed to appreciate at that moment.
ďSo, this is what youíve been doing all evening, is it? Donít you have any sense of responsibility?Ē
Jane felt black with anger inside and, without thinking, pushed her daughter toward the desk.
#1 What does wild grace mean? Does wild in this case mean the painted bird looked wild or Maryís painting skills are undeveloped but powerful and attractive?
#2 Is is it correct? I think it should be isnít it. But she was speaking so she made a mistake, I think.
#3 What does felt black mean? Did she feel gloomy? Did darkness settle over her thoughts?
In this sentence the 'wild' refers to the grace. It describes the natural grace of the bird, rather than the cultivated, practised grace of a ballerina, for example.
2. You are right that normally a positive statement is followed by a negative tag question. However, in this example a postive tag is quite correct. Using a positive tag here adds a dimension of sarcasm, shock, anger, disbelief. The mother is clearly annoyed!
Compare "He thinks he's funny, doesn't he?" (I'm asking you to agree with me) and "He thinks he's funny, does he?" (I don't think he's funny at all, and am surprised/angry that he thinks he is).
3. The whole phrase is "black with anger". Don't separate the phrase, then the meaning is clear.