The purpose of The Witch of Blackbird Pond includes exploring the experience of being an outsider, which we see both through Kit and Hannah. But just as Kit challenges the puritanical values of her relatives, they challenge hers, for her family owned slaves, she does not know the value of work, and she seeks to be admired more than to find a more authentic identity. This she finally learns through Hannah and the beauty of nature (bringing in the relationship of transcendentalism to puritanism). The purpose then, is to break down boundaries of designating any of these people and the values they embody as either this or that (as either good or bad) but to preserve the complexity of various ideas.
The Witch of Blackbird Pond is a story about Colonial America, the hardships and the controversies. The protagonist is Kit, and her position as an outsider is helpful for showing reader's an outside perspective of the community. Although the community believes that they are living by a strict and proper moral code, and that they are protecting themselves about what they perceive to be dangers (witches, etc.), through Kit's eyes we are able to see that these people are prejudice and paranoid, and prone to rash action.
There are many themes with this book, but the over-reaching purpose is to show a complete picture of colonial America - both the good (Kit and her family) and the bad (the community).