You need to remember that this poem is part of a series of poems that are elegies for Hardy's dead wife, Emma. They are characterised by a sense of deep lament and also regret for the way that Hardy treated Emma during his life. In this poem we are presented with a man who is haunted by the supposed "voice" of Emma. He doubts it is her, but then says if it is he wants to see her as she was in Cornwall in the earliest days of their courtship:
Can it be you that I hear? Let me view you, then,
Standing as when I drew near to the town
Where you would wait for me: yes, as I knew you then,
Even to the original air-blue gown!
Thus the "air-blue gown" is a compound epithet that is mentioned as an effort to remember and resurrect Emma as part of the excitement of the first two stanzas, which, however, quickly give way to the despair of the third and fourth stanzas as the speaker realises how utterly alone he is in his grief.