The Dragon Ball Wikia doesnt list any transformation for Fu, but this more than a power-up looks like a transformation. The black aura alone could be a powerup, but his eyes turn red and his hair gets spiky similar to a super saiyan. Does any manga or videogame show Fu have a transformation or it's just a powerup? It's not only a semantic issue because a transformation allows the user to boost his power beyond the limits of his base form. I dont think there is anything official about this, but I think it's worth mentioning just in case, there are many fans already calling this "super Fu" or "super saiyan devil".
That's the form called Super Mira as the name suggest it was first used by Mira by and since Fu is Mira's son i guess he can use it too.The form itself is just Super Saiyan but mutated by demon DNA.I'd link the wiki but i can't so try finding it as "Super Mira" there.