A black mirror is a very helpful device when it comes to being able to distinguish values on a form. Historically, the black mirror was an object used by landscape painters to simplify the values in a scene. It was a convex, pocket-sized, blackened mirror called a Claude glass, after artist Claude Lorraine. You can see the effects created by this little gem here on the PetaPixel website. I first read of the black mirror on an atelier student's blog. It's been so long, I can't remember her name.
A black mirror reduces the values of the scene it reflects and makes it easier to discern the distinct light and dark values when drawing or painting them. This is especially useful on colored objects where the values may be hard to see with the naked eye. You can test this by looking at your cell phone screen when it's turned off. See the distinct areas of light and dark in your face? Now, look in a regular mirror. Those lights and darks may not be quite as obvious.
Some people paint the backside of a piece of glass black to make a black mirror. This is a good way to make it, but I found an easier, safer way and it works great. Simply frame a piece of very black paper in a picture frame with glass in it. The frame gives you something to hold on to and protects your hand from the glass. I used an 8 x 10 inch frame, because that's what I had lying around at the time. There is no need for it to be that big. You can use any size frame you want.
Here's a picture of the balls I set up to paint as reflected in the black mirror. I was having a lot of trouble seeing the distinct shadow forms on the balls, but using the black mirror really makes them stand out.