Maybe it was a deliberate creative decision,lol.

Anyway So that would not be the first time people have messed up the saber colors. The Revenge of the Jedi Teaser poster by Drew Struzan had vader holding a blue saber and Luke a red. He can hardly be blaimed that all ILM gave him were black and white stills from empire strikes back. Still had he even done his research on the first movie where Kenobi has a blue sword and Vader a Red he might have not made the blunder.

Had the videocasette come out at this point in time when he sketched the poster i do not know. Maybe in late 1982 tape players were expensive or something. And Star Wars was only out as a rental only tape.

Since i have not played the Full Force Unleashed game maybe those scenes were flashback scenes when he was a good guy before being taken on as vaders apprentice after vader killed his father. I don't know if when he was a boy and a jedi padawan having a blue saber would make sense. But if it is when he is older working for vader it makes little sense. Unless he is doing a contract killing or something and wants it pinned on the Jedi only sith use red sabers. Maybe a red saber would draw suspicion on Vader. Only Vader and the Emperor are the Sith left alive in the universe.