- A reference guide and manual in Russian for Heroes 3.

You can't apply a blessing, just as you can't apply a curse, in my opinion. And of course, you need to consider the damage spread and the ratio of attacker's attack to defender's defense (for specific examples).
Therefore, luck*1.5, and double damage*2 = in total, the base damage is multiplied by 3 (strangely enough, I didn't know exactly how much luck increases damage until now; by the way, I don't think there's anything about it in the reference guide).