I take her descriptions of the Crawfords as "brown" (Mary) and "black" (Henry) to mean that they just aren't as fair as the Bertrams. In Pride and Prejudice, Caroline Bingley insults Elizabeth Darcy by calling her brown (she was tanned from the sun). I believe a lighter skin tone was generally considered more genteel, and the Crawfords just happened to be a bit darker, as some people naturally are. The point I gathered from her descriptions of the Crawfords is that they don't fit the ideal descriptions of beauty for the time - but have other qualities to recommend them.