To me, sex link means you can tell the sex at hatch. There are other sex links characteristics, but these usually don't show up until later, like the pointed hackle feathers. To me, those don't count. And you need to be able to see the characteristic and not have it masked by something else.
With those two, Black Rock and Light Sussex, all chicks should have black down. The Light Sussex will have the Silver gene, so you have half of the requirements for Red Sex Links if your LS is a hen, but the Black Rock would not work as the father. You'd need a red or buff rooster, like the Rhode Island Red, New Hampshire, Speckled Sussex, Buff Orpington, or many others.
For Black Sex Links, the hen has to be Barred and you need to be able to see the white spot on the chick. The Black Rock could work as the father, but the LS is not barred. Those will not make Black Sex Links.