The style of Hardy's "Darkling Thrush"...?

It's not exactly unique; Hardy's verse style is within the mainstream of late-Victorian poetry; you might compare this poem to other poets of the period, such as the pre-Raphaelites, Symonds, Gerard Manley Hopkins, and Christina Rossetti. In terms of themes, the poem is reminiscent of Wordsworth's calmer nature poetry (It is a beauteous evening, To a Cuckoo, etc).