Hold on a second. "Darkling" isn't a word, is it? No, no it's not. (OK, we're fudging here. A loooooong time ago, way before the poem was written, "darkling" was sometimes used to mean "a creature of darkness." Sort of like an evil spirit. Except not.)
So why the heck wouldn't Hardy keep things simple and call this poem "The Dark Thrush"? Or maybe "The Darkening Thrush"? Or what about the simple but ever-appropriate "So This One Time, I Heard a Bird Singing"? (A thrush, by the way, is a kind of bird.)
Well, we can't answer those questions for you. But we do have some theories about why Hardy would deliberately use an antiquated word in his title.
1. Hardy's a big fan of the Old Ways and of Tradition. Notice how his poem is set in the country and not in a city? That's because folks used to live in the country…but now they live in cities. See? For Hardy, old = better. And that could apply to the poem's title, as well. Want to indicate that the bird is bringing joy to a dark, dark land? What better way to do it than to use old words in new ways?
2. Hardy is an early Modernist. Hardy's often been described as one of the first Modernist writers in England. Why does this matter? Well, it helps group him with other folks who happened to be interested in similar uses of language, like T.S. Eliot, whose poem "The Waste Land" used so many obscure phrases that he eventually wrote his own footnotes in order to explain what they all meant. Hardy's not nearly as crazy as that. (Phew!) But he does share that Modernist interest in bringing old words back to life. Wait a second….bringing something back to life? Doesn't that sound sort of like what the England of Hardy's poem needs? Well, yes. Yes, it does.
3. Hardy is connecting his poem to Keats's "Ode to a Nightingale." The word "darkling" also appears at the beginning of the sixth stanza of John Keats's totally famous bird poem, "Ode to a Nightingale." Maybe that's where Hardy got the original idea? There are actually plenty of similarities between the poems – the whole bird thing, the speaker's despair at the state of the world, lots of emo going around – but Keats writes his "Ode" at the beginning of the nineteenth century (1819, to be exact), while Hardy writes his exactly eighty years later, at the century's end. We can't say how deliberate Hardy's references to Keats's poems might be – but it's quite a coincidence.
4. Maybe Hardy just likes looking down his nose at other people. As in, "you don't know that 'darkling' was once a word? Hmph. Shows how much you know."