It's not that uncommon to get chicks with unexpected coloring, especially with mixed breeds like black sex links. It's just the fluke of genetics that sometimes non-dominant genes from the mother mixed with non-dominant genes from the father combine to express those hidden genes in an offspring. And sometimes you get sports, which are color gene mutations. These mutations brought us all the color varieties we now know from the original wild type coloring of ancestral red jungle fowl.
