I agree with what's been said in this thread so far.

An admin of a large wiki shouldn't be allowed to unblock themselves, if another admin blocked them.

However, on small wikis, this would lead to a first-mover advantage situation, so admins should be forbidden from unblocking themselves if there are more than a certain number of admins (and bureaucrats).

I would recommend a threshold of five admins. Notice that if there are only three admins (with Nemo's proposal), if one admin blocks another admin, the situation reduces to a "shoot first to win" between the two remaining admins. If there are five admins and one blocks another, there will still be three uninvolved admins left to argue it out :)

The Cantonese Wikipedia recently came close to a situation where an admin might get blocked for bad behaviour. A few users presented a strong case that an admin had been acting against policy. Because we have a dozen admins, a few other admins were able to discuss the matter, and issued strong words of admonishment to the unbehaving admin, and the unbehaving admin disappeared from the wiki since then. Thinking back, one of the concerns we had was that an admin could unblock themselves anyway, so there was actually no real course of action to take other than desysopping (we have a bureaucrat; not me). If this feature of "no self-unblocks on wikis with lots of admins" was in place, then the threat of a block would have had more teeth.