I see a problem with the current power sets, what if I want to do a melee darkness wielder? I mean yes I can take like martial arts and darkness powers but there only blast power in the darkness set. It is also applicable other set such as fire and ice too. I want stuff like Shadow Punch, Blade of Darkness and the like would that be doable?

I haven't looked at the entirety of these power sets, but I would assume these will come down the road. I do know that you can find power replacers, or craft them, that allows you to add different effects to you existing powers.

Power replacers can make ur hands and weapon glow and look different, its also possible to change the colour of other powers... so your sword swing can leave a purple trail if darkness is the look you want!

Like others have said you can color the attack trails blue/purple, and you can get power replacers that add a darkness-type glow to your hands and feet when you attack and have a chance to apply a darkness blast on any Unarmed hit.
You could also get Ego Blade attacks and color the blades themselves blue/purple to look like a blade made of darkness.