Shades are primary colors mixed with various percentages of black.
In this series of yarns, I've mixed a basic, bright red with black, in amounts from 10% to 90%. All of the dye mixes are the same 1% depth of shade (dos), and use 56 ml of dye on a 120 yard, 2oz. skein.
Here you can see that a bright color will get toned down pretty quickly with the use of just 10% of the black, creating a beautiful range of maroon, raisin and rich dark chocolate-red colors. The lesson is that the deep black should be used in very small amounts if you are just trying to tone down a too-bright color. There are other options to do that as well, for example, using a complementary color, or an already muted primary, such as indigo or brown.