zahavasdad, its an interesting topic. I know for many chassidim preserving their look is extremely important. I mean they have been wearing the same clothing for literally hundreds of years. I would think this to mean they would want to preserve the color of their offspring as well.
Perhaps there are two social groups within chabad. The ones with yichus dont marry out. And the BTs do not care about race at all.
While I could be wrong, I do not think you will find the kiruv chabad Rabbi marrying his daughters to the BTs he brings in. Which of course could present problems.