Blake is teasing us and won't say who it is...but I've got  a couple of guesses.

At first I thought Darius Rucker- The ACMs tweeted out this pic which looked like a bald man but a closer look shows a hat on backwards?

So my guess is Luke Bryan.  Blake joked about this person being unattractive.  He said the artist is a fan favorite.  All adding up to sound like Luke.  Or this could be wishful thinking :)