I looked up what Cuckoo Marans chicks look like and are similar to Barred Rock chicks with that white spot on head. As for a BLack Star coming from a Barred Rock hen the males have the white spot like a BR chick and female Black Star chicks don't. I've got some pics of my chicks around here somewhere...
Regardless, any cross of heritege will be easier to sex as the male chicks will look more like hen and females like roo. A true sex link (I prefer to call em Star) is a no brainer situation when looking at freashly hatched chicks- Males will be white in case of a cross with RIR roo to white hen or in case of BR hen will have the white spot on head.
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Yes they will be a Black Sex-link.
The chicks will be either Black with a White spot on there head (cockerels) or all Black (pullets).
For the most part most any color Male over a Barred Hen will, "create" a Black Sex-Link.