One is the name of a dog, been dead for nearly a hundred years.The word when used as a name for a dog is not offensive, it can be offensive when used with the intent of being offensive
Some people find the expression "black man" to be offensive
The other is a family name still in common and current use.
The owner of the dog was a known racist. Even if his views were acceptable then, they're not now.
There's nothing offensive as describing a Black man as a black man. Some prefer that terminology.
This description in this article, of some offensive terms frequently uses the term black men and black people.
Terms You Might Not Know Are Racist
Some racist terms have long been included in the American vocabulary, and many people are clueless about the origins of these words.
www.thoughtco.com