I'd say that the report of a black P-51 as published in that "Checkertail Clan" book was probably a case of misidentification. That kind of wrong ID was pretty common from what I gather.
Erich is right when he says that the captured fighters were operated by Zirkus Rosarius. They were used for evaluation and mock combat against operational German fighters. I think the actual designation of the "Zirkus" was 2.Staffel Versuchsverband Oberkommando der Luftwaffe, or something like that. (Sorry if I have that wrong. My high-school German is way beyond rusty these days!)
It's my understanding that KG 200 engaged in mostly clandestine operations and long-range recce work, yes? Am I also correct in saying that they used Mistels late in the war, or no?