"How to make a GameObject ""Black""?" (706340)

I’m making a character selection system similar to Crossy Road. I would like locked characters to be almost completely black like in Crossy Road (see image below).

The problem is that my characters are made of multiple prefabs so they use multiple MeshRenderers that may use multiple different materials that may use multiple different colors.

Is there a general effect I can use to make the GameObject and all its children darker?

Generally speaking the solution is to replace the material on all objects with one that uses a solid black unlit shader. I guess you could also plausibly use a projector with the same solid black unlit shader.

Does it need to be a GameObject, though? Couldn’t it be just a texture?