Blacks natural speech patterns were formed by those who raise them,just as a baby raised in the south will have a southern drawl.By the time a child enters school they have for the most part already developed a speech pattern,and for many blacks,people in thier home dont speak ''proper'' english.Remember that blacks come from several generations of people who were held back educationally(it was ILLEGAL for a black person to read at one point),and put more emphasis in hard work than pronouncing words correctly
I can speak ''proper'' english but its not my natural speech pattern.I learned proper english in school but I learned my speech pattern from those I was raised around,who learned it from thier parents,who were raised down south in the 20s and 30s,who learned thier speech pattern from slaves/or the children of slaves.
If I had been raised by whites my natural speech pattern would be like thiers.If I had been raised by spanish speaking only parents I would speak spanish.And so on.
One can learn to speak a new language or develop a new speech pattern but it wont be thier natural,relaxed way of talking.