Working on a character selection screen and trying to get a shader/mask over the characters for a locked look. Looking for help on the approach.

I'm working on a character select screen and I'm trying to give the locked characters a shader dynamically visa code. I basically want a shader or mask to go over the sprite in order to make it look locked. I don't want to have to go through and create separate sprites for each. (I have around 16 characters right now) All I basically need is to have an image/texture show up with the same shape and size as the character but have it be all black with a "?" at the center. Right now the problem is I can't get any of the shaders to show up correctly. Using an image on a UI canvas, under the materials section, the Image gameobject keeps showing up as black when I use a shader. I'm still not sure if this is the right approach, do you guys have any suggestions? I could just make a "general" image for locked characters but I think it would look more polished if I kept the shape of each individual character. Anyways let me known what you think. I can post some pictures or give more info if that helps.