I feel like Bloodmary sounds better. Rolls off the tongue easier. Granted, it also sounds akin to Bloody Mary, so perhaps that's why the author went with Maryblood instead. It's a fine name regardless.
@Svelandria I was thinking about that as well, and I wonder if "Bloodmary" sounds better to us precisely because we're already familiar with "Bloody Mary." Like we have an established order in our heads for the sounds in those two names, y'know?