I'm wondering if part of the reason why some of these comic books featuring black characters did not sell well was because they were too specifically written as black people from those backgrounds, instead of just a character who happens to be black.
For example, Black Panther had the African backstory and very specific African culture. Not everyone might have necessarily been able to relate to that in the US. Luke Cage was written as someone from a poor neighbourhood in Harlem. Again, that might only appeal to a certain percentage of the readers.
On the other hand, are some of the white characters written specifically as white or as neutral and just happen to be white? Are they facing specifically "white" issues or just issues that every person faces, whereas maybe some of the black characters had issues that were too culturally specific?
Had Black Panther not been an African king but just an agile black guy living in New York who decided to don a costume and call himself Black Panther, he might not have been far different from these other street-level costumed fighters like Daredevil and Spider-Man. Would he have been more popular and been able to sustain a book? I don't know. He might have had a broader appeal though because he might be more of an everyman than mainly appealing to a certain type of audience.
I think you pose some interesting questions. Personally I don't care for the term 'happens to be' whatever, because it makes it feel like it's an accident, and racial categorization is something that isn't an accident in this country.
While some of the first, and subsequent black superheroes were probably created not only to add diversity but to address racial inequality, I don't see how that makes them too culturally specific for mainstream readers (i.e. whites) to grasp, especially today. It's funny to me that so many comic book readers get Asgard, Atlantis, Galactus, Kirby's Fourth World, etc, but can't wrap their heads around or find some commonality in black human characters. Perhaps there is a fear or annoyance that black characters are going to be preachy or talk about racism, and it's something they don't want to hear.
Some of those characters did address those issues back in the day, but that's something very rarely done today, and if it is, it usually confirms with the very acceptable mainstream 'personal responsibility, bootstraps' lines. Yet I wonder why there seems to be this either trepidation or disinterest where black characters are concerned far too often.
I think far too many whites buy into this belief that they are raceless and objective, that their culture is universal. I'm black. I don't think that is the case, but it doesn't stop me from reading, enjoying, and supporting a lot of white cultural products, including many comics (including some comics that feature black characters). I think there is enough commonality in the white experience that I can get it, even if I don't agree with everything within that culture. I have to wonder if some of the whites who are recalcitrant would just try to reach out a little more they might find something different than they were expecting.
As for your Black Panther example, we do have instances where Panther has removed himself from some or all of the trappings of his Wakandan heritage and that didn't result in an increase in his book sales. Granted, going along with your line of argument, perhaps the 'damage' had already been done in the minds of readers so they weren't going to accept him regardless.
But if Black Panther had just been an 'agile black guy living in New York', I still think he would have encountered the same issues. I mean, some would charge that he's a rip off of Spider-Man or Daredevil, ergo they aren't going to support him. Also, Marvel already have black characters like Luke Cage, Falcon, Night Thrasher, etc. and none of them have done as well as the African Black Panther at being the solo headliner in the a book.
Also, a black American adopting the name Black Panther could lead to a whole host of other racial issues due to the likely connotations some readers would make with the real-life Black Panther Party of the 60s and 70s.
I don't think we can escape race. I think we have to deal with it. And I find nothing wrong with characters that reflect that.
I do take issue though at times with white creators' ideas of what a black character is, should be, etc.
In the 60s and 70s, that was the best you were going to get in mainstream comics and I applaud many of those efforts. But some 40 years later I still think mainstream comics is dealing with issues of diversity on the pages and behind them.
As for the Human Torch news, it struck me odd at first. And then I thought it might work if he's adopted. I'm sure they wouldn't have a black Sue and Johnny Storm. All that being said, if Jordan gets the role that's cool with me, though personally I would rather see a push to develop more black superhero films instead of just changing the race of white characters. In this instance I don't think Johnny Storm is really all that race/culture specific, but still it would be great to get a Black Panther, which might result in an increase in his comic book sales, his status within the company, and his merchandising. Ultimate Spider-Man, Luke Cage, and Blue Marvel could work for me. Don't think Storm or Brother Voodoo would work as a solo film and because they've screwed over Monica Rambeau over the years I would say no to a solo film, but I think they could all have some cool parts as ensembles. Same with Night Thrasher. I haven't seen how Don Cheadle fares in Iron Man 3 yet, but at least the groundwork has been seeded for a War Machine spin off if Marvel wanted to go in that direction.
On the DC side, John Stewart, Black Lightning, Static, Icon, or Mr. Terrific in a solo film would be great. And another crack at Steel or Vixen in the Justice League movie (sorry but I don't think she can carry a solo film unfortunately). Independents like Miranda Mercury and Black Jack are out there too and could make excellent films.