Just to clear out the doubt, because I also had it, the blessing that both armors have is not a real Ashera blessing.
When in the prison cells map in PoR, Sephiran has the skill Mantle, which is the equivalent to Deghinsea's and his blessing in Part 4 Endgame of the RD game, which is the real Ashera Blessing.
In PoR, we see that neither the Black Knight or Ashnard has such skill, which leads me to believe they have a minor blessing of some kind (pretty probable Sephiran's magic or something granted by the Galdr) that reduces damage of most weapons to 0, except Ragnell.
As for the origin of the armors, I support Sephiran's doing.
Zelgius worked under Sephiran ALL the time; as the Black Knight, he approached King Ashnard and gave him the armor with the blessing both armors shared.
As for Gurgurant, that sword is a belonging of Ashnard, it was no gift from Sephiran or Zelgius. Remember, even if Ashnard says his dragon, his armor and weapon were given by the Black Knight (FE 9 conversation), the dragon and weapon were already under his command when he secured the throne of Daein slaying his family and any other rightful heir to it.