I almost always use black in a piece, especially a portrait, but I know from teaching, that students can overuse black and muddy their colors, so I had opted to leave it completely out.... which was a mistake.
with black added:
To be fair I also did darken the crease on the right eye and lightened the highlight right below the left eye. BUT look at the difference! I added black to the background and blended out over the dark brown and dark red on the face. I also added black here and there... especially in the eyes and eyelashes.Adding black enlarges your value scale. Your eye sees thousands of values... so many that the painter (or drawer?) can not create as many values as the eye can see using pigment. If you take away one of your precious values from the scale, the depiction of light in a piece will suffer. The darker the value you can get on the dark end of the scale, the lighter the highlights will look on the other end and more the sense of light in the painting (usually something most realists are after). The same is of course true for a really white white. (My favorite dark dark black is Derwent Coloursoft black and my favorite white white is Caran d'Ache Luminance white.)
I know its subtle but the top value bar (below) is a cross section of the girl's forehead: one with black and one before I added it. Which one do you think has more depth?
Why else is black important? Well for one - just like a complementary color does - black can really make a color pop against it or stand out (if used in a background), but if you use black, it is a bit less jarring. Take a look at the squares below. The green and red seem to actually vibrate while the red on black is less jarring, but the red still stands out. You can of course get the same effect by using a darker green for the background... by adding a bit of black to your green. ;-)
Don't worry about using black. What I do, and have my students do, is save black for the last step of layering. First build up your color, then add complements to darken the dark areas, and lastly black for the very darkest of darks: usually right under an object in a still-life or the pupils of the eye in a portrait (for starters). :)
& hey you've probably been using black all along and didn't know it... I mean what do you think they add to black grape... or black cherry?
I remember hearing one of my professors say that the exclusion of black from the pallette started with the impressionists and it has stuck... if anyone finds a really good article about that - let me know!