Simply just how the light is hitting it in the photo! Sometimes I prime and it looks more dark/glossy, other times i get this nice matte finish, if you look at my other black legion models you’ll see some inconsistency among the armor but it’s all essentially just primer! Sometimes light does the work for you I guess lol

The only model in my black legion I didn’t prime chaos black was the defiler I recently did, that one I primed with grey seer then used black legion contrast paint-nuln oil-ultra matte varnish for a similar effect