Cobalt Blue is a villain in DC's New Earth.
Malcolm Thawne was one of the twins chosen randomly and stolen at birth by Dr. Gilmore who reported him as a stillborn to Dr. Henry and Nora Allen. The reason Dr. Gilmore gave Malcolm away to the Thawnes was because he accidentally strangled their child while drinking heavily. Malcolm Thawne is the biological brother of Barry Allen, otherwise known as the second Flash.
Hugo and Charlene Thawne were a corrupt and vile family who raised and abused Malcolm. Hugo and Charlene were using a strange inherited Thawne power, a blue flame, to heal people, afterwards selling jars of petroleum jelly with blue food coloring to the healed and their friends. All the other members of his family had this power except for Malcolm which was why they would taunt him for his failure to handle it.
Malcolm eventually learned he was adopted and located Dr. Gilmore. Gilmore told Malcolm the truth about himself and Malcolm killed him for having picked him to be with another family. Malcolm started to go traveling and got into constant trouble with the law. Malcolm learned his brother Barry became a forensic scientist and got a job as a janitor in the Central Police Police Station where Barry worked. Malcolm started to hate Barry for everything he had that could have been his own if it hadn't been for chance and even witnessed the accident that made him the Flash.
Malcolm confronted his foster grandmother to learn more about the blue flame that the Thawne's had. Malcolm's grandfather trained him on how to use the flame to steal everything he desired as well as how to place his hatred for Barry inside a talisman that he wore on his chest.
Malcolm then became a super-villain Cobalt Blue and started to go after Barry for revenge. Flash eventually defeated Cobalt Blue with his own blue flame.
After the death of Barry Allen, Cobalt Blue did terrible things in memory of his brother: he blew up the Flash Museum, destroyed the house Barry used to live, destroyed Barry's laboratory in the CCPD station, and stole Jay Garrick's speed. Barry and Iris' nephew Wally, the third Flash, tracked him down and acquired enough speed force to defeat Cobalt Blue while making Cobalt Blue reveal his life story to him.
Cobalt Blue sparked a family duel between the descendants of the Thawne and Allen family that would last for centuries until a truce was declared. However President Thaddeus Thawne was enraged to learn that his daughter was married to a member of the Allen family. Cobalt Blue is also also the ancestor to Eobard Thawne, otherwise known as the Reverse Flash or Professor Zoom.
In Other Media
Malcolm Thawne doesn't appear in the Flash, however a character named Eddie Thawne appears instead as the ancestor to Eobard Thawne and is Barry's friendly love-rival for the affections of Iris West. Like Malcolm, Eddie works at the CCPD station where Barry is and is prone to jealously like when he punched Barry in the nose for trying to ask Iris out. It was speculated by fans that Eddie was the long lost brother to Barry and that he was stalking him in the first episode, however that fortunately didn't turn out to be true meaning that Eddie never became Cobalt Blue. Unfortunately, Eddie died in the season 1 finale where he shot himself to prevent Eobard from coming into the world. However, some fans believe that Eddie Thawne will on day come back to become the villainous Cobalt Blue so that he'll destroy Barry Allen.