The black box will be explained, but it does suck its only been mentioned twice out of 250 chapters and the only character who knows how to access it, is also the antag who just so happens to be the strongest character. No past sorcerer knew, no Kenjaku, no Shoko, no post prison realm Gojo, no Yuta, no Higuruma (hyped up sorcerer since debut).
Just seems really poor to not expand on it just a bit more or give someone else it.
At least with barrier-less domains we get it explained somewhat and Kenjaku also has it, at least with Black Flash its explained and developed on its worth in the world.
This is like if Kubo mentioned Bankai then only gave it to Yamamoto for 500+ ch and showed us a second of it then later on goes "very unknown no one knows only strongest does yippee"
Why even make it a concept if you won't expand on it soon and then decide to rush your ending?