Well, for a start DON'T use Black straight from the tube. Black will totally finish off your painting. Straight from the tube it has no depth, the light doesn't bounce back off it - basically it just falls FLAT/DULL/WITH NO IMPACT......
This is where you need to MIX YOUR OWN BLACK
I hope this will show up from the photo on my camera...
Black from the tube
So from now on always mix your Black
Another alternative to Black is
Alizarin Crimson + Hookers Green
TEST THIS OUT FOR YOURSELVES,
YOU WILL SEE WHAT I MEAN.