The body parts referenced in this poem – hands, eyes, shoulders, and feet – are examples of synecdoche. Synecdoche is when a part of something is used to refer to the whole thing. For example, when someone yells "All hands on deck!" he doesn’t actually mean that he wants a bunch of severed hands on the deck; rather, he wants the people and their hands to help with the ship. So, the phrase "immortal hand" references the whole being or person that the hand belongs to, while at the same time focusing on the hands as the means of creation. The eye is representative of the whole body and person, but also focuses (ha ha) our attention on the faculty of sight.
Also, by including only parts of the creator in the actually poem, Blake contributes to the mystery of who or what he actually is. It’s like having only a few extreme close-ups of a person: you can see the hands, shoulder, feet, and eyes, but you can't see the whole package, and that means you can't even tell who you're looking at.