In some ways, this poem is like an object lesson in what to expect from a ballad. It starts in the middle of the story, there is an ambivalent ending, it has (lots of) structural repetition, there is a conventional four-line stanza form, ABCB rhyming scheme, archaic language and the depersonalised figures typical of ballads.
On the other hand, Keats makes it his own. there are many factors that individualise it, from the Shakespearean references (to Macbeth? Romeo and Juliet?) to the single-ended frame narrative. Did he copy that from Coleridge's double-ended frame narrative for The Amcient Mariner, do you think?
Anyway, one of the best ways to understand Keats (or Coleridge) and what they were trying to do by imitating ballads is to go and read some for yourself. This website will show you a fine collection. It is an archive of Child's Ballads (Francis Child was one of the most influential collectors of popular ballads in the 19th century. Have a look and let me know which is your favourite. How does 'La Belle Dame Snas Merci' compare?
