The issue isn't that he's an alien. It is that he's a rural farmboy from the heart of Trump country. And it isn't just some small part of his backstory. Clark Kent's upbringing in rural Kansas has always been a major part of his character taking up a large chunk of time exploring it. His race would absolutely be an issue in that environment.
Now, I'm not saying it can't work as an alternate take on the character with a bunch of major changes, but it would be a very different Clark Kent. He isn't like a Barry Allen or Hal Jordan where his race would have no impact at all.
I would say it goes a little beyond that. At its heart, the story of Superman is an immigrant allegory. In its original form, a specifically Jewish allegory, though you could reconceive it. The problem is. . . ultimately, the African American cultural experience *isn't* a tale of immigration, its a tale of slavery. Making Superman black ( and implicitly African American, because everyone in pop culture always ignores the more recent black immigrants and their own separate culture ) would change his underlying premise enough to make him, effectively, a different character ( see also: Icon ). This is why, by contrast, the Hispanic take on Superman from Justice League: Gods & Monsters *does* still work, since its fundamentally still an immigrant allegory, even if what that immigrant experience means is different.
( As an aside, the main reason why I would look askance at changing Hal Jordan's race, is not because Hal Jordan has to be white, but because you have Green Lantern's of pretty much every race already. Why change Hal's race when John Stewart is *right there*? Otherwise, of the Big Seven JLAers, while Superman probably should be white, and Batman *definitely* needs to be white, most of the rest are flexible. Flash, Green Lantern, and Aquaman can be whatever, and Wonder Woman can be pretty much anything other than "East Asian" or "Aryan Poster Child". Martian Manhunter. . . theoretically could be anything, but he's pretty much been adopted as black for decades, and I'm leery to change that. )
