A black sex link is SUPPOSED to be a hybrid cross between a rhode island red (or production red) rooster and barred rock hen(s).
When I crossed my RIR roo with my BR hens as a kid, the FEMALES were always black with red in the breast and had BLACK legs, whereas the MALES were always black-and-white barred with YELLOW legs.
Since sex links are a hybrid, they won't breed true and you may see a variety of color patterns in the offspring. But when I mated my black sex links together (years and years ago), I got a good number of pullets that were black with silver in the breast, like your bird there.
-Anne
When I crossed my RIR roo with my BR hens as a kid, the FEMALES were always black with red in the breast and had BLACK legs, whereas the MALES were always black-and-white barred with YELLOW legs.
Since sex links are a hybrid, they won't breed true and you may see a variety of color patterns in the offspring. But when I mated my black sex links together (years and years ago), I got a good number of pullets that were black with silver in the breast, like your bird there.
-Anne