Why is it so ironic that the buccaneers have cut a black spot from a Bible to give to Silver? What does the black spot represent? Why does Silver mock the man who has the Bible?

The dreaded black spot is never a good thing to get. It represents the crew no longer has belief or use in the person it is given to. When Pew delivers one to Billy Bones, it contains a demand for him to appear before his crewmates. Silver is more rebellious and mocks whoever dare to cut into a Bible. He mocks the hypocrisy of a scourge using a black spot to threaten him.