Today, I'm going to give you guys the history of Black Flash which won't take long because well, there's not a lot to talk about. But it sure is interesting.
Black flash first appeared in flash issue 138. Black Flash is basically death for the speedsters and those who are connected to the Speed Force. Its direct origin has been debated. Many think it was initially created as the embodiment of death, as traditional death was not fast enough to capture those aligned with the Speed Force. Several writings however have revealed that it's a dark aspect of the Speed Force. The Black Flash has appeared in numerous occasions, primarily before the death of a speedster.
It has appeared prior to the deaths of Barry Allen and Johnny quick. Max mercury has also seen Black Flash as he has had several near-death experiences. When the Black Flash eventually came for Wally West it proved unsuccessful inadvertently bringing Linda Park; Wally West's girlfriend and soon-to-be fiancé into the Speed Force instead. It tried again but this time froze time to those outside of the Speed Force. Due to this fact, Max Mercury, Jay Garrick and Jesse Chambers were able to aid Wally in battle. He was able to beat the black flash by racing it to the end of time where death has no meaning.
The Black Flash would reappear once more but this time Bar Allen who had taken up the mantle of the Flash, after the events of Infinite Crisis. In Final Crisis it was revealed that the Black Flash is the same being or possibly an aspect of the black racer, who is death for the new gods. In flash rebirth, Barry Allen begins to convulse and transform into the new Black Flash. Barry soon starts killing other speedsters through their connection to the Speed Force. Seeking to contain the Black Flash, Barry runs back into the Speed Force causing Black Flash’s essence to be re-absorbed by the force. It was then revealed that the mastermind behind Barry's transformation was none other than his arch enemy, Professor Zoom. The latest incarnation of Black Flash is seen during the Darkseid war. During the battle with the anti-monitor, Darkseid summons the black racer to help them in a fight.
For those of you who don't know what the black racer is, he's basically a grim reaper for the new gods in the DC Universe. Anyway the Anti-Monitor is able to merge black racer with Flash making Barry Allen the new black racer and god of death and/or the new interpretation / version of black flash and I must say his design looked awesome.
As for powers and abilities, the Black Flash is rumored to be as fast as any of the Flash heroes and capable of running at least four times the speed of light. So since he's that fast, he's considered to be one of the fastest beings in the DC Universe. He is capable of freezing time for those outside of the Speed Force. It could be argued that he is capable of drawing non speedsters into the Speed Force after he was able to mistakenly bring Linda Park into the Speed Force. and like I said it has been shown to possess a host like it's done with Barry Allen.