I've seen so many darlings of technorati die sudden and surprising deaths, I really think it is safe to point at most social media tools and predict a death date of less-than-2-years.
And you are right - things like Black Twitter are real, and vibrant communities, that have adopted the tool, and use it heavily. But these niche interest communities probably don't even match the size of Twitter's staff. Just because they are visible above the waves for the moment, doesn't mean they will stay there and continue to use the platform. Switching is basically free, and all you need is a small core of trend-setters to move to GrbLiXpaff (or whatever), and suddenly nobody will touch Twitter with a 10-foot pole.
It happens really fast, too.
Twitter is trying to innovate, so it doesn't appear stale, but each new feature treads on other apps that already own that territory. (Vine, or Snapchat, or Instagram, or FB, or whoever.)
I shall dance upon its digital grave when it dies, too.