How do the black flowers initiate a discussion on hidden sins in chapter 10?
Chillingworth is pretty convinced that Dimmesdale is the father of Pearl, and he wants to test his theory by drawing the minister into discussions of hidden sin. He sees black flowers growing out of a grave in the cemetery, and he mentions them to Dimmesdale, saying that they might have grown out of the deceased's heart, possibly because of the person dying with unconfessed sins hidden in his heart. This causes Dimmesdale to defend people with hidden sins, making possible excuses for them.
This is Chillingworth's way of letting Dimmesdale know that he suspects Dimmesdale as being the one who had the affair with Hester Prynne, Chillingworth's wife (although no one in the town knows yet that Chillingworth is really Mr. Prynne).