Walk in darkness power shows crossed swords for double damage when you attack. Is that only for Melee Weapons or is it for any attack (ie Musketeer or Witchdoctor weapons)?
The crossed swords is your Weapon Power. It applies to all weapons, for they are weapons, and Weapon Power is the strength of your weapon. So yes, you may use a wand or gun, and get double the damage you would normally do. The power of your weapon can be seen in the Your Character section under the Pirate Hook on the top of your screen. I hope I helped.
P.S. If I'm wrong myself, please point it out so I can be correct.