Cool running makes sludge, so your hot running is good. It may be graphite or assembly lube. There is always some oil left after a change, and graphite or such will continue to pollute your oil. Sounds like you're doing the best things possible, especially draining the factory fill early and using a good filter. Getting dark means it is holding things in suspension, should be good. With hard driving your rings/cylinder walls should have been well fitted together by now. It should take only a couple hundred miles of correct break in to get to a stable ring/wall condition.