I've never heard of them eating crabs,maybe large adults do in the wild...Give it a few days to settle and try dropping some raw shrimp in the tank.It may take a while but when it gets hungry enough it should take the shrimp aside did.
Never had a Knife turn down Frozen Bloodworms, but you mentioned that it's only been 4 days. Give it some more time. I'm sure it'll eat soon. You can also try Ghost Shrimps. Small crustaceans are probably found in larger specimens.