I'd be questioning the authenticity of that website, as they called a Black Sex Link a cross between a Barred Rock and a Rhode Island Red. The pic they have is of a Barred Rock. MY Black Sex Links were solid black!
https://townlinehatchery.com/product/black-sex-link/
Black sex link males ARE barred and their pictures are a decent representation of the typical black sex links.
Not every black sex link develop the rusty red feathers on the head and neck but most do.