As far as I'm aware, the nuance between the two terms would lie in that they blocking specifically refers to it from a character perspective. Can I see what they're doing? Can I see what their face while they're emoting? I think it's more to do with live theater than television or film. For example, before rehearsals you might "block out a scene" where you read your lines with your counterpart while standing in the correct position and planning out when you need to be where.

Composition on the other hand, would be directly related to the camera and where things are to make a good shot. They overlap for sure, but blocking I feel is more limited in the scope of what it can refer to.