There is one difference between the two cases. If you are vampire and take atronach blessing, only the blessing is added and since necromage is presented, it is boosted. When you cure yourself of vampirism, necromage is not the only thing removed, your race is also changed. When race is changed, it is possible (maybe not) that the blessing will be recalculated. That means the same logic can not be applied to the cases and to know for sure, you need to test in-game.