All I can find on the web is this critique of the poem:
The speaker, in this poem, speculates on whether he is or is not like the fly, carelessly swept away by the speaker's hand. The fly may have little, if any, conscious awareness of himself and his mortality and, if the speaker shares that freedom from awareness, then life or death is of little consequence. If, on the other hand, the fly does have human awareness then it is a different story.
Blake was a highly religious man with a unique take on heaven and hell that I am not even going to dare to get into lest I sink in a sea of complexity and flounder there.
Good luck, sorry can't be of more help.