Marketable animal means what?
Something that fools an order buyer?
If you want a marketable animal for the "I love black cattle" crowd, I think someone would be well served to get a F1 cow as unrelated as reasonably possible to the black breed of your choosing that you want to use for your terminal sires. Or, maybe a cow that is a first generation of a three way cross.
Think ... for a black calf ... to have cow that is 1/4 continental, 1/4 african, 1/2 Indian ... and bred to a nice Angus bull ... you'd get polled black calves out of hardy, maternal mamas.