WARNING: The following contains spoilers from Titans United #4, on sale now.
In Titans United #4, by Cavan Scott, Jose Luis, Jonas Trindade, Rex Lokus and Carlos M. Mangual, Blackfire's capture of Superboy results in him being brainwashed into a new version of Black Zero. This name carries a long history tied to Superboy's, but one that spells disaster for his friends. Black Zero isn't just some enemy of Superboy from the past, he is Superboy.
A Superboy whose history diverged from Conner's just enough that he is more powerful and far more twisted. Though this isn't that same character, Conner using this name is still nothing to take lightly.
Black Zero debuted in The Kingdom #2 (by Karl Kesel, Tom Grummett). He was an alternate version of Superboy in a Hypertime stream. In this branch of history, Superman never recovered from his fight with Doomsday and the Superboy clone was allowed to reach full maturity. Thus, when he awoke, he had all the powers the younger Conner would not, as well as his tactile telekinesis. He became this world's Superman, but quickly lost public approval when his inexperience resulted in many people losing their lives.
This turned the populace against clones in general, blaming an entire group of people for the mistakes of one. Seeing the violence perpetrated against his kind, the clone took on the name Black Zero and used Cadmus to create a clone army that he then conquered his Earth with. He eventually expanded his mission to other Earths where clones could face oppression, attempting to conquer them, yet always finding himself opposed by his counterparts across Hypertime.
Black Zero became a successful threat to his heroic versions, easily outclassing them one-on-one. His status as a fully matured clone meant he had powers that Conner had yet to manifest, giving him an edge in combat which he used to capture his alternate selves. So for Blackfire to have captured the main Conner and experimented on him all this time implies that she is attempting to recreate this powerful figure in the young Superboy. She had access to Cadmus tech, so it is entirely possible that she learned what Conner could have become if he had been allowed to mature.
Now, Conner has returned to Earth as one of Blackfire's soldiers, seeming to take the initiative on her will and confront his friends. Superboy was already one of the most powerful beings on Earth, he could easily fight on a level playing field with someone like Starfire or Donna Troy. So, if Blackfire did actually enhance his powers to what they could be in the future, then the Titans aren't going up against a corrupted version of their friend, but an enemy whose powers are on par with Superman. A level of power who, if it is anything similar to his alternate self, is entirely capable of killing the world's heroes.
Even more concerning is that he may not remain on Blackfire's leash for long. It is unknown what she did to brainwash him in so short a time, but there is still some free will left, as he acted without orders to confront the Titans. He may end up turning on Blackfire and becoming this universe's new Black Zero, triggering a war between himself and the rest of the world.