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...
No, they're not sex links. Only a solid colored rooster over a barred hen will give you black sex links. BSL hens don't have the gene for barring. The reason some of the chicks have head spots and others don't is because a BSL rooster has only one copy of the barring gene, unlike a pure barred...