I'm currently in a dark mood, but I want to turn that into something constructive. Any advice on how to do that? Has it ever been done?
Just write all the thoughts you don't think. There's no other way to do it. Let your fingers do the work.
Thought the statement was a bit obscure myself, too. Edited something in, hope it helps. Otherwise I will be with you shortly.
Apart from what you are feeling, do you have something to say? Have your emotions affected your life, your work, your relationships. Have you done anything to cope or deal with these effects. Have the dark feelings given you any new insights? Do you see your friends and acquaintances in a different light? If you are in a dark place, do you have any ideas on how to get out of it, or has anyone offered you help and advice that may change your life down the road?
Make a villain character and have him/her say all the nasty things you'd wish you could say, but know you shouldn't; or things you'd know you'd never say out loud. Have then rant and rave, choke a hapless mook, whatever you can think up of. Let your anger flow through you. Give in to your rage. *ahem* Just write out your frustrations through a character. Sometimes when we're angry, that's when the best villains are born.