I haven’t found a way. I also seem to have come into that problem (I have 9 blogs at wp.com, several of which I rarely use), as such, I think the best thing to do, would be to make a big sticky post, saying that you are no longer using that blog, and to contact you if said person wishes to have it.
I believe there was another forum thread a couple of years ago. I think judyb12 may have posted it and it was being used to collect the blogs that members were willing to transfer to other members. Forgive me for not searching for it but I’m dead beat.