In The Witch of Blackbird Pond, what is the secret Kit told Mercy?
The answer to this question can be found in Chapter Four of this great story and the secret concerns Kit's true reason for coming to New England and leaving sunny Barbados so far behind. Kit tells Mercy in a sudden moment of confidence that she was expected to marry an elderly man who was a friend of her grandfather's. Her grandfather had apparently ran up huge debts and so there was a very real pressure for Kit to marry this man in exchange for forgetting those debts. Note what she tells Mercy:
He used to come often, and afterwards I found he had lent Grandfather money, hundreds of pounds. He didn't want the money back--he wanted me to marry him. He tried to make me think that Grandfather had wanted it, but I'm sure that was not so. He wanted to pay for everything. He would have kep the house for us to live in. Everyone expected me to marry him. The women kept telling me what a wonderful match it was.
Therefore we discover that the principal reason that Kit has come to New England is that she is escaping an unwelcome marriage proposal from a man who is much older than she is. Even though she is finding her new life in New England very challenging to begin with, it is still much better than the alternative.