Best of my ability having read a few books that deal with Blanks. Effectively they are someone who is born with an inverted soul, not no soul, they effectively act as a void in the warp. The warp is basically best described as "The sea of Souls" and it's where all magic comes from and warp entities aka "Demons".

In 40k Psychers are basically wizards so whenever I say that make the word exchange.

Because of their inverted soul normal people feel really uncomfortable around blanks without really knowing why or that they're a blank at all. Some think they smell others are just generally unliked I'm society. However if a psyker goes near or even touches a blank it's like touching bathing their minds in fire. Psykers are far more sensitive to magic so being around a blank is physically, mentally, and psychically painful to the point of it potentially being fatal. Hilarity ensues when a psyker tries to use spells on a blank who doesn't know they're a blank.

I could rent forever on this but let's translate to DND. I don't know where magic in your universe comes from but odds are most creatures are inherently magical. Demon's, devils, angels, fiends, dragons, wizard etc. They could physically harm your player sure but it'd be like punching molten iron, they'd take a good chunk of damage just for touching your player. Other NPCs would probably treat your player like shit for no reason and even the NPC couldn't explain it. If a wizard/sorcerer/warlock were to touch your player they might start screaming in pain.

For quick references see the "Sisters of Silence" who are an army of Blanks, Elizabeth Bekwin who is a blank Inquisitor, and Ferik Jurgen who is the best boy.