I have seen photos of pure velvety black bettas before. I've also read in many different places that pure melano blacks cannot be line bred because the females are inherently infertile.
Yep, melano females are infertile. Black lace and a relatively new line called double-black are not, however. Double-blacks are the darkest of the three varieties, with melanos being in between and black lace the lightest. Melchiresa in my sig is double-black.