Unfortunately it's not possible without unblocking them
I have several design that is owned by someone I blocked,t this person used to be fine but has recently been bullying and harassing me and making up rules left and right. So I don't want to unblock them but I would like to see what they are doing to my designs
even if you do decide to unblock the person and see what they're doing, once a design leaves your hands there's really nothing you can do, most users on the forums agree that once that design is sold that's pretty much it, it's the person choice to do whatever they want to it (unless the user in question saw and agreed to your TOS in writing, then you could consider action)
I personally don't recommend "checking" and just leaving them blocked, but it's up to you of course (/lh)
You'd have to unblock them to check on it, and I don't recommend doing that. Honestly, once you let go of something, you shouldn't be checking on it anyway.