I would have happily linked these remarks to impressions and Anticlimax and any of the other commentators who doubted whether Hardy (the figure in the poem) finds any "blessed Hope" in the thrush's song.
I read the final stanza (one complex sentence) as expressing Hardy's apprehension of the irreconcilability of the "ecstatic" song of the thrush with the surrounding corpse-like world. The first two stanzas detail Hardy's vision of the world, and the third places the song in that world and describes it. The final stanza considers the song and the world together. Hardy characterizes the irreconcilability of the song and the world as best he can: he says that nothing in the world could "cause" or account for the song, and so (a) one might postulate that the thrush has an inner awareness of a "blessed Hope" that allows the thrush to create the song, and (b) Hardy "could think" that he hears in the thrush's song this blessed Hope, the awareness of which by the thrush is required for the song. It is not Hardy's intention, I suggest, to say that he hears, even dimly, the blessed Hope in the song or that as a result of the song he is willing to postulate (however credulously) the existence of the blessed Hope, Introducing the blessed hope into the poem is simply a means to express the incomprehensibility of the thrush's song in the world: it is so incomprehensible that a listener "could think" (I can't improve on Hardy's choice of verb here) that he or she hears in the song a blessed hope the maker of the song is responding to.
I am grateful for the remarks of Robert Pinsky and the commentators.