pastel and black pastel are two completely different genes.

pastel makes the light areas of the BP lighter and more yellow, overall it makes the BP lighter.

Black pastel makes the dark areas darker, and overall darkens the whole BP.

You can do beautiful stuff with both genes, but they really are completely different.

if you combine pastel and black pastel, you get this:

http://www.worldofballpythons.com/morphs/black-pewter/