Thanks so much guys. Hope it didn't come across as me showing off, rather than making a very subjective point in a very subjective debate.
The gods of schematic agriculture love you! What helmet is Blackwall wearing? I like it! Actually, what is Cassandra's helmet, too? I thought it was a Free Marches helmet at first, but no, the visor for that one doesn't have a base like that.
Blackwall is wearing Rainer's Helmet / Knight Helmet, from the JoW DLC. It's obviously unique to Blackwall only.
Cassandra is actually wearing a Free Marches Helmet - the base I think you're referring to, is the chinguard of the armor's chestpiece. I dyed the metal and the leather straps to match those on her armor. The geometry of the helmet and the chinguard match perfectly for some reason. That was actually the rarest I'm glad to have found - a Tier 3 schematic for Cass with her awesome "coat"-style armor.
P.S.: I know there's probably a more historically accurate term for what I call a "chinguard", but that's what it looks like to an idiot like myself...