Well, in fanfiction where Barry Allen turns into Black Flash, it's a major transformation. Firstly, his powers could become more unpredictable and dangerous. He might not be able to control his speed as he used to. Secondly, his friends like Iris, Cisco, and Caitlin would be horrified. They would try to find a way to bring back the old Barry. Emotionally, Barry would be in turmoil as the Black Flash entity takes over. His speedster friends from other cities like Wally and Jay might also be in danger as the Black Flash is known to target speedsters. It would be a very different and much darker story for the Flash universe.
The plot would become much darker. Instead of the usual heroics of Barry Allen, there would be chaos. His enemies might initially be confused and then try to use his new dark state to their advantage.
One of the best is 'The Flash: Rebirth'. It delved deep into Barry Allen's return and his complex relationship with the Speed Force. It also reintroduced many elements of his character and his place in the DC Universe.
One possible scenario could be that Barry Allen is exposed to some dark energy or a time - related anomaly that slowly starts to corrupt his mind and make him turn into Savitar. This could be due to his overuse of the Speed Force, which has unpredictable effects. Maybe he starts to see the world from Savitar's perspective, believing that the only way to 'fix' things is through Savitar's extreme methods.
Barry Allen became The Flash after being struck by lightning while working in his lab. The lightning was charged with chemicals, which altered his body's chemistry and gave him super - speed. This event transformed him into the Scarlet Speedster we know as The Flash.
Barry Allen became The Flash after being struck by lightning and doused with chemicals. This accident gave him super - speed. He was a forensic scientist at the Central City Police Department. His mother was murdered when he was young, and his father was wrongly accused. This event drove him to use his powers to fight crime and seek justice, not just for the city but also to clear his father's name.
Maybe the OC is from a parallel universe where the rules of the Flash's powers are different. This could lead to a lot of exploration and conflict as Barry tries to understand and work with this new element in his world. The OC could bring new knowledge about speed force that Barry has never known before.
First, give the OC a clear backstory. If they have powers, explain how they got them. For example, if they were in an accident like Barry, detail what kind of accident and how it differed from his. This will make their existence in the Flash's world more plausible.