At first, you could clearly see they wrote the black guy as the love interest for the blond, and then decided to change it to the white guy; got to love Hollywood at its finest!
Michael
http://s1.sfgame.us/index.php?rec=58163
The younger girl may have had a hankering for some Neb, but it's not apparent whether Neb wants her. Remember, he came from a time when he could be whipped at the least and lynched at the most for just looking at a white woman. Heck, some states had laws against interracial marriage as late as the 1950s. I don't think that it's wimpy that the girl switches her affections to Herbert. She didn't exactly seem like the loyal type anyway.
"There will be blood. Oh, yes! There will be blood."-Jigsaw; "Saw II"