
It said in one of the earlier chapters that the extrovert is able to see curses, they show up as black on the body part. The introvert is always chasing paranormal stuff that he’s constantly being cursed, but in some chapters it changes on thickness. So, he is human just heavily cursed :) everyone sees him without the black marks

The mc obly saw the male lead's (ml's) face black because the mc can see curses. Remeber when the mc saw the curse on the leg as a black spot. That must mean the ml has a curse on his face hence why mc sees his face as black. No one else can see the curses like the mc can so everyone just sees the ml as a normal human.
he didn't think himself as paranormal at all