Actually his new lore describes how he looks under this armor
" a shadowy form appeared upon the slab. The deathly shade rose to its feet, leaving the skeleton behind.
It was a wraith formed of pure darkness, yet its eyes burned with malice."
So basically he is a living shadow under that armor,his color text supports that
The Iron Revenant stepped in to end the fight, but the knight turned aside his strike with exquisite skill, and rammed the point of his blade into Mordekaiser’s chest with all his strength and weight.
With a wrench of metal, the blade punched through the breastplate above the heart. There was no resistance from within, as if the suit were hollow.