I transferred my one book successfully to my Kobo account, where it appeared as an Adobe DRMed epub. So that's nice
Interesting too that it's not in Kobo's catalog so even though it's in my library it can't be purchased from Kobo. Which is also a good sign.
I would have bought more from Blinkbox if I'd known I was going to be able to free them. Well maybe. They weren't as cheap as Amazon generally.