_ am I a happy fly: metaphor
_ some blind hand: symbol (of death) or a form of synecdoche (part for the whole)
_ thoughtless hand and blind hand: qualifying a hand of "thoughtless" or "blind" is a figure of speech, but I've forgotten which one, sorry. I suppose it could be a hypallage?
_ the hand brushing away the summer's play is also figurative, because the hand is brushing away the fly, not the summer's play. Metonymy? Not sure...
There are other figures of speech (apostrophe, for instance, at the beginning of the poem; chiamus I thee thou me), but I'm not sure whether they come under the appellation of figurative language.
Well... a few - simple - thoughts:
The poet compares himself to the fly because like the insect's, his existence is insignificant: mortal (he too will be brushed off by a hand, in time) and devoted to apparently frivolous activities ("I dance and drink and sing") which will be interrupted just as brutally as the fly's "summer play".
You would expect him to mean, also, that he is to the fly what God is to man. So in way, there is a double identification here through the idea of death: the poet is like the fly, but he is also like God, or whatever divinity brings death. This could be construed as slightly vain on the part of the poet, and rather blasphemous (does God too, like him, drink and play and sing?). But it also gives an idea of the connections existing between all the realms of the universe. In a way, it brings to mind the medieval world-view: you know, with the microcosm reflecting the macrocosm etc.
It could be, though, that the meaning of the poem is not as simple as it first looks: there are several things that are difficult to understand:
_ note the shift from "thoughtless hand" (the poet's) to "blind hand", which suggests that what brings him death is not God - how could the conventional, omniscient God be described as "blind"?. So what is it? Sorry, I don't have time to look up blindness and see what could be behind this word.
_ note the fact that if the poet compares himself to a fly (a little cliché, so not unexpected), he also compares the fly to a man, and this is stranger and more surprising.
_ the last two stanzas are more complex: he assimilates thought to life, and lack of thought to death, and true enough, his thoughtless brought death to the fly. But it can also be interpreted differently: lack of thought, for a human being, is death too. (I don't see if you understand what I mean, I have trouble expressing my own thoughts on the subject!) You are alive only if you think.
_ The connection between the last two stanzas: he will be a happy fly whether he lives or die, ie if he thinks or not, it won't change anything to his state of mind. I can't help thinking this is rather ironical, but then again, I've never studied Blake in depth, so don't know whether it's an uncharacteristic thought for him or not.
http://www.online-literature.com/forums/showthread.php?t=39002