well, its tough to say, I asked the same questiuon a while back. I think it could be from the blast. or it could be because he is more human now. but then again,he says he will destroy anyone who messes with the earthrealm, so it could be he has changed not just on the outside but inside.

when you train with Raiden in Outworld (right outside of Lei Chien) he is in his darker outfit (as opposed to how he looks in Orderrealm in Konquest earlier on) and he is called 'Dark Raiden, the Thunder God Reborn' so I am thinking it is the rebirth process that makes him look as he does.