For generations according to the French site the BC were mixed with the Wheaten Marans, I do not know if we will ever get rid of the Wheaten influence involved with the BC. I can say one thing that I have done and that is not use the light colored leg BC, also the white feathered Males and the blended Hackle Males. I have eggs hatching next week and will start posting pictures on my profile page once or twice a month for a while to see how they grow off.I always think of the males as being more like 75-90% of any breeding program, because as you said, they are half of every chick you hatch and so spread their genes around a lot more than a hen does. That's one of the reason I bought so many birds to start off with, so I would have lots of boys to choose from.
I also think you are right on the blue mixed into black; but also when other varieties are mixed in. Look at the mess we have with wheaton cropping up everywhere and I have something that might resemble black tail buff (posted those boys on the general marans thread last week). One of the reasons when I started out I tried to find folks working with just the BCM and not mixing in blues. I know there are things several generations back in most lines, but at least I could avoid things crossed in recently.