Very true, but as Harry often says, when you work magic you have to mean it, so I would definitely be taking a very close look at the character of the dying wizard before allowing it. (there's also the possibility that magic powered by death might have an unpleasant edge to it that you might not want attached to your blessing)
In game terms, I'd at a minimum expect a character to be tagging one or more aspects relating to the target of his blessing, and I would certainly be looking for any compellable aspects that would interfere with his selfless act. Someone who has aspects relating to self-sacrifice and love for another character who is in immediate danger? A death blessing seems appropriate. Someone with aspects like "vengeful" and "quick-tempered" who tries to cast a death blessing on someone is going to have a much harder time.
Dude, their giving up their pc, that odd's are thieve been playing for more than a few sessions unless you run short shots. They die weather they succeed or fail. It makes sense to allow the player one crowning moment of awesome at their death. Rather than "Oh, you flubbed your dice because you were at an aspect negative, your spell fails and you die. Wasn't that touching?"
(I tend to take a less mechanical and more narrative approach. So I guess it differ's with style, But I place PC sacrifice on a very high pedestule provided it wasnt brought about through player stupidity. If the party wizard is struck down by the fae lord at the climax of an arc, and is gasping his last breath to protect someone he cares about, or "bless" one of his allies who are his best friends. Ill let him do it, He's paid a entire player character to do so. No higher price can be paid than that, and with his death all of his subplots, his roleplay, his everything. Go away. It's a capstone on the character.)
