At the end of season two of The Flash, season-long villain Zoom was taken by Time Wraiths as punishment for manipulating time in his travels through the multiverse. As this happened, the speed force changed Zoom's face. He became something else - the Black Flash.
In the comics, Black Flash is one of several characters who is a personification of Death. Specifically, he is Death for those with enhanced speed. Whether he is a hero or a villain is never specified. If you're a speedster and you see him, you're dead. He's not a murderer - rather a force of nature who comes for everyone - even heroes.
It now appears that Black Flash is returning to The Flash, and other shows as well. The Wrap reported on showrunner Andrew Kreisberg speaking about the character's return at the Television Critics’ Association’s Winter Press Tour.
"I will say that you might be seeing him on multiple shows... [Black Flash] is a classic character. We have storylines on multiple shows that fit having him around. There’s something so scary about him, something even scarier than when he was just the man, because now it’s now he is the Grim Reaper, he is the devil. How do you stop the devil? So it presents an interesting challenge for the heroes of the respective shows in which he appears."
Kreisberg is obviously referring to The Flash's Arrowverse sister-shows - Arrow, Legends of Tomorrow, and Supergirl. It's certainly not uncommon for these shows to share characters. Small crossovers are common through the seasons, with big ones each November, like the recent 4-episode event Invasion. Recently, two villains first introduced on Arrow (Malcolm Merlyn and Damian Darhk, as well as the Flash's arch-nemesis Eobard Thawne/Reverse Flash) have all teamed up to form the Legion of Doom on Legends of Tomorrow. So Black Flash showing up on the other shows wouldn't be an unprecedented occurance.
In the comics, Wally West defeated Black Flash by racing him to the end of time. On TV, only the speedsters Wally and Barry are fast enough to do something like that. (Supergirl might be able to as well, though her powers come from the yellow sun and alien DNA, rather than the speed force.) In any case, the heroes, sidekicks, and legends on all these shows have no ability to fight back. So if he is not just after speedsters, they will have to find another way to defeat him or someone will die.
There is also a question of casting. Teddy Sears played Zoom, meaning he was Black Flash when the character was first seen. Kreisberg has talked about bringing him back to the show before. But now there is likely a scheduling problem, as Sears has been cast on 24: Legacy and may not be readily available to rejoin the Arrowverse.
The Flash season 3 returns to the CW on January 24.