What does Aunt Polly mean when she calls Tom's supposed like a "blessed lie" in Chapter 20, and why does she call it that? What does she eventually discover about his lie?

Ridden with guilt, Tom apologizes, but tells Polly that he had meant to give her the piece of bark. Confused, Aunt Polly asks Tom: "Did you kiss me, Tom?" Upon hearing him admit that he did so because he loved her, she forgives him and sends him off to school. Aunt Poly is so touched that she thinks Tom's lie was from a good place.