Hi Monica, i think black is one of the hardest colours to do, too much grey and it washes the black out, not enough and you dont get the effect. when i mix my pallet for any colour i put the lightest colour and the darkest colour next to each other and then mix them to gether forming various shades so that i can work the colour quickly. You have to imagine that the light is coming from directly overhead so if you get a crease right at the top would be lighter than the bottom, and anything underneath would be even darker. So from top to bottom, light to dark and the shades in between. For this black i mixed a bit of blue with the black for the base coat i could then use straight black for the deepest shadows
Heres a few pics of the finished rear right half
As you can see the very top of each muscle is the lightest shade and the bottom is black. Hope this helps
Paul