I forgot to mention a couple of things you may already know. A buff rooster over a barred hen will give you black sex links. The boys will have a spot on their heads when hatched, the girls will not. The boys will grow up barred, the girls will not be barred when they feather out. The exception to this is that the down color has to let you see the spot. Usually with this cross you can.
In theory that should give you a solid black pullet and a black and white barred cockerel. But real life usually doesn't work that way. The genetic modifiers that make the red an actual buff color quite often have a strong effect on black. When they feather out they will have black, but sometimes some feathers will be yellow. It can be quite striking. I'd love to see photos of yours after they have feathered out.
@Mosey2003 , how did yours turn out when they feathered out. Any photos?
To make red sex links the rooster has to have gold and the hen has to have silver genetics. The Buff male will have gold, the Barred Rock hen may have gold or she may have silver. Often they do have silver but it is not guaranteed. You also have to be able to see the difference in the down at hatch. The pullets will have reddish down and the males will have yellow. Genetically there are different ways to make black too, that can play a factor. So while it is possible you might see some difference in the down, especially on the face, the typical black genetics of the Barred Rock hen makes it unlikely.