Alan Bligh smiles and says nothing.
What do you think. It clearly is a Loyalist with its lightning bolts for the Great Crusade. He is hunting or Avenging people as per the names on his right gauntlet.
My immediate theory is, they are black because they are in mourning. We know the Emperor possibly came to some agreement with the Chaos gods to help his Primarch project. He must have tried it before he gave up and got the Chaos gods to help.
Maybe the first Primarch died / could not be awoken. He would have tested the Gene templates before he decided to start building the Legions. Maybe these are first-ed Astartes? Could this be the first image we have of Legion 0?
Black with Blue eyes is Iron Hands. But there is no other motifs that back this up. Nor is there a silver gauntlet. I would also by the same reasoning say it might not be Raven Guard as they have red eyes.
It could be the paint was stripped off the suit and it was a Iron Hand. The armour is battered and damaged so could be a Legionnaire from Isstvan. The Iron Hands lost their Primarch, but I don't think it would turn a whole host to change their iconography etc.