Black sex link is a name for a mixbreed that can be sexed at hatch. 50/50 chance at either. Male chicks will have a white dot on their head, female chicks will be solid.
Black sexlink a are a specific type of cross that allows for easy, reliable way of sexing the chicks at hatching. Barred hens can only pass their barring gene on to their male offspring. When you cross a barred hen with a non-white, non-bared rooster, you get black sexlinks. Only the males will have the white spot on the back of the head, indicating the presence of the barring gene.