Blue is only mentioned in the series and has yet to make a physical appearance. However, as many character retain their sprites from the game, it can be assumed that this would hold true for Blue.
Blue is only mentioned once in the series when Rusty checks his Twitter page and sees that Red has posted something new. He reads it aloud and discovers that Red recently defeated Blue in a pokemon battle. This inspires Rusty to seek out his own rival to push himself to be the very best.