Black is the darkest color group you'll ever see, since it is considered the absence of light. You can set all of the RGB values to 0 but that will give you limited options for blacks. If you want an off-black, it may be straightforward to start with a gray and make it really dark but you will probably want to tweak the really dark gray to make it look more appealing.
In fact, there are actually many kinds of off-blacks you can make but you have to keep the RGB values low. Dark brown-blacks and blue/purple-blacks are great choices for off-blacks. Black is often a reflective color so it often will be biased towards a color that it is reflecting.
The highlights tend to bring out how dark the surface actually is. Warm and cool colors in general are great for accenting full blacks. Duller blacks tend to be lighter and brighter blacks are darker.
The above rules mostly apply to background colors. But for regular-sized text, you'll almost always want to go with the full black for the sake of contrast. (Obviously you can't do that with dark-colored backgrounds like this, where you'll want full white text instead. But for lighter colored backgrounds, like what you see on the other pages, you will want the full black.)