There are a couple of differences between red and rich black velvet cake.  First of all, the color!  A red velvet cake has red food coloring added to get a deep red color, while a black velvet cake is black from black cocoa.   Secondly, while both cakes are chocolate flavored, a red velvet cake is made with regular unsweetened cocoa powder and black velvet is made with a form of dutch process cocoa powder that is black in color.