African-Americans, as slaves, got much of their English from their masters and overseers, many of whom spoke in the dialects they and their ancestors brought from 'the old country', hence ax for ask, etc., pointed out above.
Spelling was standardized largely through the efforts of Samuel Johnson, the 18th century lexicographer and his followers, but until formal schooling eroded the dialects, the old words persisted. And still persist among those of us who enjoy the language in all of its parts.

Always wondered why 'am't I' never became popular for Am I not.