How Sacrament works is that it adds its non holy damage to your auto attacks. Then its holy damage component is a separate auto attack that only deals that 40 + your magic attack (I think, I might be wrong on this point). Blessing adds to both of these. If you gain another attack from Enchant Fire from a Pyromancer, Blessing will add to that as well.
The next important factor to look at is the attributes. Sacrament only has one that increases your dark resistance while active. Blessing however has one that adds +2 to blessing damage per attribute level. Max that and you gain another 100 damage on Blessing. Suddenly Blessing seems much more attractive.
Basically it means when you swing an auto attack once with Blessing and Sacrament up you gain the damage of Blessing twice.