Can I play blessed alliance after damage resolves to make him sac a creature I didn't kill?
Yes this works how you want it to. Note however that the creature will die after it has already dealt its combat damage, so if you are trying to stop it from hitting you this would not work.

The combat phase has 5 steps:


Beginning of Combat
Declare Attackers
Declare Blockers
Combat Damage
End of Combat


A creature becomes an 'attacking creature' during the Declare Attackers step and remains an 'attacking creature' until the Combat phase is over (or it gets removed from combat some other way). Thus you are able to use Blessed Alliance during either the Combat Damage or End of Combat steps to have your opponent sacrifice a creature that survived the damage dealing part of the Combat Damage step.


  508.1k Each chosen creature still controlled by the active player becomes an attacking creature. It remains an attacking creature until it’s removed from combat or the combat phase ends, whichever comes first. See rule 506.4.