ABCB
This means that the first and third lines rhyme, and the second and fourth lines rhyme.
For example, the first stanza of the poem goes:
* The wind was cold, the stars were bright, (A)
* The birds were silent all, (B)
* And in the dark blue vault of night, (C)
* The watchful owls did call. (B)
Noyes uses this traditional ballad rhyme scheme effectively to create a sense of rhythm and musicality, which is important in a poem about a romantic and tragic story.