Why does it have to be the darkest of brothers?
You do realize AA are a product of admixture already? And we have the lowest IR rates already so I'm not sure how you can misconstrue that as "whitening". A Black person having a Biracial baby who has a baby with a White is more "whitening" than a Biracial having a baby with a Black person. If anything, it's you who are promoting whitening, whether aware or ignorant. You let Biracials have their own group without them having babies in Biracial unions then your lineage is going to be looking like this