It's hard to say for sure. Sometimes stories are a blend of truth and imagination. You'd have to look into the background and author's statements to determine if The Porter has any basis in reality.
The porter in Macbeth provides comic relief through his humorous and irreverent dialogue. His presence lightens the otherwise intense and tragic mood of the play.
I'm not sure specifically who the authors of 'Eugene Black' stories are. There could be many independent or emerging authors with such works, or it might be a relatively niche or regional set of stories not widely known in the mainstream.