The Black Flash is the representation of Death for DC 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, or if it was simply a manifestation of itself, much in the same manner as the Black Racer. 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, but wasn't caught by the creature.
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 as the very presence of it corrupted time enough for this to occur. 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, but you can't kill Death.....
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, damaging him enough for Batman to defeat later on.
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 Black Flash simply disappeared once more.
When Bart Allen was dislodged from time after trying to escape the grasp of Brainiac, he becomes connected with the Black Flash and is sent throughout time and space, being forced to drain and kill the likes of a young Max Mercury. Eventually making it to his grandfather, Barry Allen, as he attempted to undo the effects of Flashpoint. Knowing that his father simply didn't have enough power to do this, Bart choose to sacrifice himself and his Speed-Force energy so that Barry would be able to do such acts. As such, Bart faded away, much in the same style as Barry originally did during the Crisis on Infinite Earths.
Eobard Thawne/Professor Zoom was also referenced as the "Black Flash" when he was resurrected as a Black Lantern during Blackest Night: while the name may fit his black costume at the time, Thawne was not a actual Black Flash, only being one by name.
Black Flash appears in Rebirth, but his mission is to eliminate people using the Forces in general like Strength, Sage, Still, as well as speed, managing to successfully kill Psych, which was enough to actually save Barry Allen as it gave him enough speed of his own to escape (this is due to Sage and Speed force being incompatible with each other)
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 well as travelling fast enough to catch Wally West at his fastest. As such it can be considered to being one of the fastest beings in the DC Universe, as even Wally struggled to escape it.
Capable of freezing time to those outside of the Speed Force, through even those who are within the Speed Force can still be effected by it if they haven't been using it for a extended amount of time.
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, but this seems to be something it isn't usually capable of wanting to do as it shows no interest in people who don't show aspects of the Forces within them.
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.
In Other Media
Smallville- A version of Black Flash also appears in the comic-only format of the TV show Smallville, namely season 11 and was the main antagonist of the storyline "Hunted".
In it, Bart Allen finds that he is being stalked by a creature that he can only see at incredibly fast speeds: so fast, in fact, that not even Clark can see it for any longer than a few seconds at the very fastest he can go. Eventually, they find that this creature feeds on only the fastest beings alive, with Bart being one of them. It is even implied that Bart gained his super speed in the first place by the creature causing a "Speed Storm" which kills most normal people and leaves them a husk. They track down a person who survived the creature's attack called Jay Garrick, who informs them that he only survived due to breaking his leg and losing his powers: for Bart to live, he must never use his powers ever again, as the creature is only alerted to his presence when Speed Storm powers are used.
However, Bart refuses to let the creature kill more innocent people while he stays by the wayside, and attempts to fight it with Clark, now with a developed suit that allows him to comprehend the being and fight it. However, Clark is no match for the brutal creature, and eventually Bart, having built up speed from all around the world from both people and other Speed Storms smashes into it, overloading the beast but killing himself in the process, leaving a giant symbol of the Speed-Force in the ground. It is assumed that both him and this Black Flash are dead, through this simply could be due to the Black Flash finally making contact with Bart.
The Flash (CW)- Black Flash also makes a appearance in the CW show The Flash, through his backstory is considerably different. First appearing in the final episode of season 2, "The Race of His Life" he is originally the CW variant of Hunter Zolomon (Zoom) but after being killed by the Time Wraiths, creatures who stalk the Speed-Force, he is transformed into a undead enforcer of the Wraiths himself, forced to hunt and do the bidding of them for all time, making subsequent appearances afterwards as the Black Flash, stalking other speedsters that attempt to time travel.