4 Of my first birds were the black tailed red sexlinks. I actually didn't even know white tails were an option until a few years agoSOME Red Sexlinks have a white tail. But SOME Red Sexlinks have black tails and do look like Rhode Island Red or New Hampshire Red or Production Red. Ideal Poultry sells some with black tails and some with white tails (according to the descriptions on the website).
The whole point of a sexlink is that they can be sexed at hatch by the color of their down. If yes, then they are a sexlink, no matter what color their tail is. The gene that controls black vs. white tails is not involved in making them color-sexable.