You might be confusing Black Panther with Blade there. Millar had pitched a Blade miniseries years ago, to be done with John Romita Jr., but the brass pressed them to do that first
Wolverine miniseries instead. He then said he planned to use Blade in that series, but then ended up not using him, but said he still had "big Blade plans." Later, Millar praised Reggie Hudlin's Black Panther, when it started, and said something about how he'd hoped to do the first "top 10 Black superhero book" with Blade, but Reggie beat him to it with BP. He did eventually write a Blade story in
Ultimate Avengers, but it wasn't all that great, in my opinion.
Anyway, now Millar is firmly working on creator-owned stuff for Netflix, and said he
doubts he'll ever write for DC or Marvel again.
But speaking of unconventional writers for BP, you know who'd I like to see take a shot at it? Warren Ellis. Secret societies, advanced technologies, arcane religious practices, that's all right up his ally. In fact, he already wrote a story about a secret technologically advanced city of Black people in
Planetary #17. This would be a natural expansion of those ideas.