You don't have to buy black paint. Instead, you can learn to make a rich, deep color that appears to be black, known as chromatic black. It may seem counterintuitive to take the time to mix it when you could just buy black paint and be done with it, but if you want realistic-appearing shadows and gradients in the deep areas of your subject, you need a black color with a little more subtlety than straight black out of a tube.
Chromatic black also blends better with the other colors on your palette, as it will be less dramatically different in color temperature than straight black because you mixed it yourself with translucent colors rather than a premixed opaque black. You'll better control the overall tone of your painting and make it more unified if you use a black mixed with colors you're using elsewhere in the painting.
A common way of creating a chromatic black is by mixing ultramarine blue with an earth color, but there are other mixtures that give an even richer, deeper black. Mix equal parts of Prussian blue, alizarin crimson, and an earth color, such as burnt sienna, burnt umber, raw sienna, or raw umber. By varying the colors a titch—a touch of more blue or a touch of more brown—you will end up with a cooler or a warmer black, respectively. These small differences can add nuance to your shadows and gradient to your colors.
When a chromatic black is added to white, you get some beautiful grays. If these grays are too blue for you, simply add a little more of the earth color to the original mixture, which will make the grays look grayer.
On a reference page, mix the following and paint a swatch with the results. Then add varying amounts of white and paint a swatch of that color in the same row, to show the differences that the various browns make in your mixture. Label the colors in the mix and approximate ratio of white in your different grays:
You can expand your chart and include mixtures using Indian red, Venetian red, and Van Dyke brown.
Mixing small amounts of your chromatic black into your colors will darken them without "killing" the color like regular black would do. Artist Jim Meaders calls Prussian blue and alizarin crimson "magic colors." Most painting teachers don't include those colors on their lists of required colors, he says, but after students discover all the possibilities of using these colors, they never go back.
That said, the Gamblin company does have a chromatic black in a tube in its product arsenal that doesn't flatten other colors it's mixed with, if you need to save time with part of the mixing step; you can still adjust that chromatic black to your liking and the needs of your painting.