The Black Flash is Death. It's that simple. It is the manifestation of death for all speedsters. It exists because these fast people are too fast for normal death-so the Black Flash gets them. Barry Allen, the second Flash, and Johnny Quick both are speedsters who are now dead. Before their deaths, the Black Flash was seen near them. Although other things killed them, those things happened because they were caught by the Black Flash.

Eventually, he decided it was Wally West's time to die. Black Flash went after him-but missed and got his wife, Linda Park West. He tried again, but Flash raced him to the end of time, where death has no meaning. It dissipated for now...

Most recently, the Black Flash appears to Bart Allen (the Flash) when the Flash loses his powers while battling the Rogues at the Getty Center. Bart is killed shortly thereafter by the panicked Rogues when it appears he would win, even without his powers.

The Black Flash was-or is-as fast as Flash. It is at least four times the speed of light, being one of the two or three fastest beings in the DC Universe