Creatures continue to be attacking or blocking creatures through the end of combat step. It is possible to cast Celestial Flare within combat but after combat damage is dealt (specifically, during the combat damage step or the end of combat step). Only attacking and blocking creatures that survived combat damage can be sacrificed at this time.

If the player sacrifices a blocking creature, any attacking creature it was blocking remains blocked. Unless that attacking creature has trample or is being blocked by another creature, it won’t assign or deal combat damage.

Celestial Flare targets only a player, not any creature. A creature with hexproof or protection from white can be sacrificed this way.