I have a scene with no lights in it (I want it lit only with one spotlight, but I want to start with it pitch black and work from there). Yet it is still lit. Most advice is to turn off AO, and I’ve done that to no effect, nor can I find any lurking nodes for this.
I think it’s because you’re not using a shader like Principled BSDF between your mix output and the surface input. When I put your mix nodes color output into BSDF base color input and then that output to surface that might be what you’re looking for.