I don’t subscribe to that theory. Few if any animals have better daytime eyesight than raptors. I suspect they can tell the difference between a crow and a chicken just fine.
I theorize that people who’ve had black chickens fare better have simply had tougher breeds that happen to be black, like heritage Australorps, or their local predators have been turned off by individual personal preference. For example, you can have two northern goshawks, and one is repulsed or spooked by black chickens for a myriad of reasons, and the other doesn’t care about the black at all, with each preference due to personality variances between the individuals.