Actually, you have something there. Anyone from the Southern States that gets into broadcast journalism ends up taking a course in how to speak Midwestern. Likewise, other parts of the country where accents are prevalent. Southerners, though, have that stigma of being uneducated because most of the country that's dealt with natives of the south have dealt with the less intelligent members. They're mostly laid back in their speech patterns.
With the black population, I wonder if it has something to do with their hearing. Have you ever noticed how loud many are in normal speech. I have some neighbors over me that are very loud. In their 50s, but when they talk, I can hear them a mile away. But, like southerners, they grow up speaking the language they hear at home.