The Black Flash is the representation of Death for speedsters and those who are connected with the Speed Force. Its direct origin has been debated to whether it was initially created as an embodiment of Death as traditional Death was not fast enough to capture those aligned with the Speed Force. Recent writings however have revealed that it is a Dark Aspect of the Speed Force.
The Black Flash has appeared in numerous occasions primarily before the death of a speedster. It appeared prior to the deaths of Barry Allen (the Silver Age Flash, Flash II) and Johnny Quick. Although they were killed by other things, ultimately it was because they had been caught by the Black Flash. It is also known that Max Mercury has seen it.
When the Black Flash eventually came for Wally West (Flash III). It proved unsuccessful inadvertently bringing Linda Park (at the time was 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 (the Golden Age Flash, Flash I) and Jesse Chambers (aka Jesse Quick/Liberty Belle, daughter to Johnny Quick who had been claimed by the Black Flash previously) was able to aid Wally in battle. Wally is able to beat the Black Flash by racing it to the end of time, where death has no meaning. For a time it remained dissipated.
The Black Flash re-appears once more this time to Bart Allen (who had taken up the mantle of The Flash, after the events of Infinite Crisis) at the penultimate moment when he was about to lose his powers while battling the Rogues at the Getty Center. Using Inertia's "time machine", they unwittingly strip Bart of his powers instead as the machine effectively drains the Speed Force from an individual. Bart fights valiantly against the Rogues, but is killed shortly thereafter fatally struck down by the panicked Rogues when it appears he would win, even without his powers.
In Final Crisis, was revealed that Black Flash is the same being, or possibly an aspect of the Black Racer, the death of the New Gods. This is confirmed by Wally West, as he and Barry Allen Use the racer to reap the soul of the evil God Darkseid.
In Flash: Rebirth, upon discovering the remains of apparently the Black Flash following its confrontation with Darkseid, 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 reabsorbed by the force. It was then revealed that the mastermind behind Barry's transformation was none other than his arch enemy, Professor Zoom. With Barry purged from Black Flash's essence, The speedster's death's status remains to be seen.
Powers & 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. As such it can be considered to being one of the fastest beings in the DC Universe.
Capable of freezing time to those outside of the Speed Force.
It could be argued that it is capable of drawing non-speedsters into the Speed Force after it was able to mistakenly bring Linda Park into the Speed Force.
It has shown to be able to incarnate itself within a host, at one point being within Barry Allen.
Black Flash seems to have a connection with other aspects of death in DCU, most notably the Black Racer.