The animousity the Flash known as Jace Allen had with his era’s Cobalt Blue was a very personal one. When he was a child on Petrus, she had infected him and the rest of the populace with a virus to end the Allen bloodline and a 700-year feud. In a last-ditch attempt to save his son, Jace’s father Blaine (the Flash of his era), froze the entire microplanet to buy time. He ultimately took his son into the Speed Force, hoping it would take him. It took Blaine instead, and bestowed super-speed powers to Jace, enabling him to shake off the virus and survive.
Jace took up the mantle of the Flash, with Cobalt Blue remaining one of his arch enemies until Jesse Quick recruited him into an all-out battle against the Cobalt Blue legacy.