Among other tools, they issued us a black mirror on the first day of painting school; it doesn't simply make the scene more pleasing, but can be a useful tool in teaching the student how to see. https://twitter.com/singareddynm/status/1340700922050195459

One of the fundamental questions faced by a representational artist is: how do I organize my values? How do I create a sense of light & form using the limited range of possible values in a drawing or painting?

It's a difficult proposition, and the muddy, dirty appearance of student drawings is very often caused by a lack of organization to your values. See the tweet below: https://twitter.com/ZacharyHundley/status/1337187509755908096

To phrase it differently: let's say you're drawing this obscure sculpture. How light do you make the bright light on the sternum? How light do you make the pecs? How dark do you make the shadow under the left forearm? How dark do you make the deep shadow between the thighs?

Almost universally, a student artist will overstate these differences--they're staring hard at the distinction between "bright sternum" and "slightly less bright pec", and are very concerned with getting that difference down. But this ends up overmodeled, like the tweet above.

What the black mirror does is to consolidate the values of the (complex, confusing) scene in reality into a simplified picture. You lose the distinction between similar darks, and between similar lights, and get something like:

I'm not saying you can't or shouldn't try to represent those subtle distinctions, but if in doubt, you're probably going to get a more powerful image by flattening broad areas of light & shadow, grouping your values. The black mirror just gives you a lead on how to do that

(you can, incidentally, get a similar effect just by squinting, and I highly recommend it. Lose details, get the big picture, always.)