It may be an obvious question, but with all the symbolism in the book can someone explain the significance of the title?
Hannah is the first witch of Blackbird Pond, but soon Kit is accused of witchcraft herself. Then, at the conclusion, we learn that Nat has named his new ship "Witch" as well. The word "witch" has negative connotations to the people in the community, but Nat transforms those to something very different, suggesting a positive spirit of vitality and change. Historically, women were named "witches" for being different in their society, and some scholars have speculated that they were often the more creative sort, out of place in societies that required women to be complacent and subordinate. Kit refuses to be those things, and in that way is very much a witch. In a playful sense, too, she "bewitches" Nat in that he falls in love with her.
This novel is very much a coming-of-age story. Kit has lived a comfortable life in Barbados, and a sheltered life. Coming to Connecticut has turned things upside for her. She faces alienation and learns about human kindness and human prejudice. Through the conflicts of the novel, she develops into an adult, more mature, more reasonable, and more worldly.
The Witch of Blackbird Pond, literally the character of Hannah, is crucial to Kit's development. Hannah provides her with the emotional support she needs in order to mature. The conflicts that arise from her friendship with Hannah give Kit the impetus to change and to grow. So the title is symbolic of the catalyst that helps Kit grow up.