Early on before CP got some of the kinks out of their new printing process (i.e. before white was REALLY white) every color was really dark on black shirts.

Now however the colors on a black shirt look about the same as any other shirt. The problem is that dark fabrics create an illusion that dark colors appear darker than they really are. This is especially true if you have a design that has no outline and a dark color right next to the black fabric especially blues from my experience. They can look almost black if dark.

But when I cover up the dark fabric with a white template cut from paper the design looks fine.

The way around this is of course to use lighter colors on black.