While this would be nice, I have to wonder about this situation:

You block someone because they are harrassing you, not becuase they're a spammer or whatever, but a legit player that just happens to be annoying. Eventually, they quit the game because nobody likes them and deletes their character. You use auto-delete, and they are removed from your blacklist. Later, they return to the game and SE restores their character (because SE likes to hold "Return to the Game" campaigns to try to lure back former customers). They are now no longer on your blacklist, free to harrass you once again.

More than auto-delete, what the Blacklist REALLY needs is a way to tag BL'd individuals - a way to write yourself a reminder as to WHY you BL'd an individual. If someone's blacklist has 198 names of gilsellers (most of whom are now inactive accounts) and two names of actual players who that player really wants to avoid and who are likely still playing, it'd be great if there could be a way to mark them as special for the next time you start sweeping garbage out of your blacklist.

A reminder tag is also a good way to unblacklist people who you BL'd for a reason which, years later, now seems pretty silly. ("Hmm, why'd I blacklist this guy...? Pulled early on a hunt??? Was I really that petty back in 2014?")