That is an interesting idea to speculate about, but given that Faulkner has no problem telling us that Tobe is black, it is highly unlikely that Homer would be so also without Faulkner's saying so.
Remember that the story is set in Mississippi in the late 19th and early 20th centuries. Their relationship was scandalous and caused gossip just because he was a Yankee. There is absolutely no way an African American man could have courted a southern aristocratic white woman without being lynched. The townspeople even gossip about whether Emily and Homer have secretly married because she buys a man's brush set and pajamas. If they wouldn't have tolerated the couple's riding around town in a carriage together, they would have all been downright apopletic about a mixed marriage.
When you read stories like "A Rose for Emily," you can't place today's morality and social values on them--although that would make a great essay topic.