I was reminded about this reading the session reports over at DF Whiterock such as this most recent one. This isn't meant as a criticism of how Bless is run there, just a look at how I run Bless in my games.
Reading other people's posts is a great way to get thinking about your game in different ways.
My previous GURPS games extensively featured the Bless spell. With two wizards who could cast it, everyone was typically under Bless +1 at all times, Bless +2 or Bless +3 occasionally (usually due to a critical or a powerful NPC casting.)
We also had a lot of characters with strong defenses, high DR, Luck, and supernatural backup plans like the very occasional Lesser Wish or Wish.
Order of Operations of Aid
The big question we grappled with as a group was, "When does Bless 'fire'?" In other words, when does Bless activate and provide its (potentially) life-saving and harm-avoiding benefits?
This was especially critical regarding Luck. It was a time-limited resource and players liked to husband it for something really important to them, like rolling to avoid death.
The question was, did Bless "know" what the attack could, or would do? Could it retroactively apply if the effects of the attack were too much? Who decided what was too much?
We decided that Bless didn't "know" anything. It didn't know if you had Luck left. It didn't know if you were going to fail a HT roll and die as a result of that attack or make it easily and live and really need that +1 or better next turn. It didn't know the damage or results of an attack. It was a supernatural blessing but it wasn't prescient.
Therefore, we decided it was the first line of supernatural defense. You still got any defense roll, if you had one, or a resistance roll, or Blocking spell, or other "normal" attempt to defeat something. If that failed, Bless would kick in and save you if the threat was sufficient to do so. You couldn't use Luck to save Bless unless you'd done the very rare thing of declaring a use of Luck ahead of the roll's failure.
Extent of Bless
We essentially made it a hierarchy - +1 shifts things one level (critical success to success), +2 two levels (critical success to failure), +3 three levels (critical success to critical failure). That made for easier rulings; it only affected an opposing success (or own failure) and made it better for the Bless recipient by the margin determined by the spell level.
And like Luck, it couldn't go back to earlier rolls. It affected the immediate roll that caused the immediate problem and went away.
How did it play?
Once we settled on this, it was fine. Bless was very useful, but its bonus would burn off pretty quickly along with its saving effect as the first rolls were failed. It ended up being a very simple ruling in the end. We found that we liked the overall effect - it was a useful spell but it didn't save you only after everything else was gone. Nicely it kept Bless +1 from being an always-on +1 to everything with a last-ditch Undo button for only the worst circumstances. It played well for us.