I'm not aware of any reputable accounts of selective breeding of African American slaves as a way to promote or remove certain traits from the gene pool. I do know that many times, especially after the African slave trade was outlawed, slave owners based their decisions on what female slaves to purchase on how fertile she was thought to be. The idea was to create a self sustaining slave population. I also know that repetitive rape was sometimes used as a way to break female (and in some cases male) slaves, and make them more subservient. Sex definitely played a role in slavery in the United States, but as far as intentional selective breeding goes, there isn't much evidence to support that.