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"Dark" is the variety. Cornish is the breed.
Just like a Barred Rock is the barred variety of the Plymouth Rock breed.
The Dark Cornish do not resemble the Cornish X that much, except in the fact many broilers have the peacombs. Originally, way back in the day, they used the Cornish sire as it's very erect posture lengthened and widened the breast on the offspring. If you look at really good Cornish, their legs appear to be miles apart compared with other breeds. They then put these on Rocks who put on thick breasts and you had a best of both worlds solution.
Now, fast forward 50 years and we can do this cross ourselves. However, the industry has far eclipsed our ability to do hybrids in our backyard. The "Cornish" and "Rock" in a "Cornish Cross" resemble neither breed in the pure form.