In "Thirteen Ways of Looking at a Blackbird," I believe the author's theme is to look beyond outer characteristics. Just in the title itself, the author encourages or suggests that there is more than one way to look at one thing. The fact that the author chooses a blackbird for his multiple observations is ironic in that a blackbird has no distinctiveness in its appearance, in comparison to another type of bird, such as a peacock. The blackbird is just black with no variation in shades of color. How ironic that the author would write of thirteen ways of looking at such a plain, ordinary bird. For this reason, it is apparent the author is challenging the reader to look at such an ordinary part of life in many different ways. Such a varied study would prove to the reader that life may seem ordinary, but beyond the typical, jet black feathers, there are interesting actions associated with the blackbird that account for at least thirteen different ways to look at it.
Perhaps your first interpretation of there being only one truth could be changed to thirteen possible truths. Why would the author suggests thirteen ways of looking at something? Why only thirteen? Why not ten as in the children's song "Ten little blackbirds sitting in a tree?" Why not "four and twenty blackbirds" as were "baked in a pie?" Wallace challenges the reader to look at one simple blackbird thirteen ways, not twelve which would at least be an even, balanced number.
Even more interesting, why does the author choose of all birds a blackbird, which has very little, if any, outstanding features? The only thing I can really perceive, infer, is that the author's theme is an underlying one of look beyond what you see with the physical eye. Since reading the poem, I am now more curious than ever about what Stevens sees in a blackbird. If this simple, yet complex, poem can so arouse my curiosity, perhaps the author's point is well taken. Look at something again and again and again and one may have a different perspective.
Stevens alludes to this in stanza II..."I was of three minds, like a tree in which there are three blackbirds. How can one person have three minds?
If you study each stanza, the reader will see that Stevens is indicating that there is more than one way to look at something. My favorite is stanza VII when Stevens questions the "thin men of Haddam" about their imagining gold birds. Stevens alerts them to the fact the the simple, ordinary blackbird brushes the feet of their women around them.
Again in stanza VIII, Stevens "[knows] noble accents, but he points out that "the blackbird is involved in what [he knows] as well. Here is it inferred that the blackbird is involved in Stevens' noble knowings. Perhaps I have given you a number of ways to look at Stevens' poem. Maybe not thirteen, but hopefully enough to help you better understand the poem.