Just because you're not assigned to go onstage doesn't mean it won't happen. If there's a chance of you having to go onstage you should be in blacks. I'm a designer, usually I show up for previews in street clothes, but if the show's going rough in tech and I think maybe I'll need to go backstage and troubleshoot, well, I show up in blacks. I agree, a
FOH sound mixer should be dressed in street clothes - and I think they should be dark grey or suchlike, non-descript and able to pass as blacks onstage.