The monitors looked grey even without being plugged in.

To be clear, this is tube technology, which was widely used right up to the end of the last century. Screens were often thicker front-to-back than they were wide. They were really heavy too, with thick leaded glass (to stop X-rays) at the front and big copper magnet coils at the back. There was a fine-pitch metal grid inside the glass, and some phosphors printed on inside of the glass. That stuff wasn't black. In operation the situation got slightly worse, from stray electrons probably, but fundamentally the displays just couldn't ever be black. It's the same as printing on brown paper: don't blame the settings of the printer when you don't get any white.