It worked for Blade three times. No one knew who the hell he was back then besides a small subset of comics fans from back in the Tomb of Dracula days in the 70s. The whole unknown thing doesn't work when you consider how D-list Blade was, and people still clamor for him. He has had a movie trilogy, his own tv series that was the highest rated on its channel, and an anime series. This all with being a character that not very many people knew about.
Also here's something to think about in regards to profit. There have been six movies featuring black superheroes as leads. I'll list them:
Steel

$16 million budget/ $1.7 million box office
Spawn
$40 million budget/$87 million worldwide box office return
Blade
$45 million budget/ $131 million worldwide box office return
Blade 2
$54 million budget/ $155 million worldwide box office
Blade: Trinity
$65 million budget/ $128 million worldwide box office
Catwoman
$100 million budget/ $82 million worldwide box office
Every single one of those movies made back twice its budget at the box office besides Blade: Trinity which only missed it by a few million, Steel which was **** on top of **** that should have never been made, and the same can be said with Catwoman. The bottom line is that people go to see black superheroes when they come out unless there is just no effort made on the creators of the movie like Steel and Catwoman. And even in the case of a really poor third outing you see a good amount of people showed up to watch Blade: Trinity because they were invested in the character at that point. Had that movie been good it would have probably continued the trend of outdoing its predecessors.
Basically people will go see something if the movie is good, and there are plenty of great black characters in all comic companies. Just look at how popular Michonne is with Walking Dead fans. No one gives a damn about her being black or a woman. They just care that she has a cool ass katana that she owns zombies with. If Marvel were to put a really good effort into Black Panther, or in the upcoming Luke Cage tv show, or even some low level guy like Night Thrasher it would sell well if it looks good enough. I think us fans over think things sometimes in regards to how much the average movie goer cares about certain things.