The blackbird is a very unconventional animal of which to be in awe. It's like being in raptures over a squirrel. You're like, "What's the big deal? It's a squirrel. We see them everyday." We have the sense that Stevens wrote "Thirteen Ways of Looking at a Blackbird" to counter exactly that reaction. In our daily lives, we fall into patterns and routines that prevent us from looking at the world in fresh ways. We forget to look at the strange beauty that can be found everywhere. The subject of reclaiming the beauty of nature can be found throughout the history of poetry of all languages, but this poem is notable because it succeeds in making an utterly conventional creature seem fascinating and even weird. To a very large extent, the mind has the power to decide what counts as amazing.
The speaker's tone is best characterized as ironic detachment.
The true subject of the poem is the way poetic sounds and images operate. The blackbird symbolizes the prototypical poet who lacks any sense of self-consciousness.