David, in my great great grandfather's writings, he mentions Negro Dogs as follows,

"On the larger farms over-seers were hired to boss and drive the negroes. Some of the over-seers were heartless and treated the slaves badly. They would tie a big man down and use the black-snake whip on him to their heart’s content. Some kept hounds trained to chase a runaway negro like they would a wild cat or a wolf. These dogs were called negro dogs. When the negroes were treated badly they would run off, but were most always caught soon"

This adds little to what you have already said, but it does place this mention of Negro Dogs in Attala County, Mississippi. Whether the dogs he mentions were Mastiffs or Great Danes, I do not know.