A message from TFN
This list is not in any particular order (thanks to darth_sarubious for the suggestion).

1. black disguises those hard to clean blood stains
   by Rebel

2. It's just so slimming
   by Jedi_Brent

3. To soak up sunlight when he visits New York. Every little bit of warmth helps when the winter is as cold as Hoth.
   by Luke 3:1-22

4. Cause when a scarred, pasty white, shrivelled up husk of a man, black will still look sexy on you.
   by Matev Kenas

5. there were no white plastic left after ten years of making armour for clonetroopers!
   by 2powerfull

6. goes nicely with the dark side!
   by 2powerfull

7. Space camouflage. Black. Obvious.
   by Boinga

8. 'What Sith' says it's this year's colour for the Universe Dominating Badass
   by Luthiena

9. He's Amish.
   by Rin, Destroyer of Keyboards

10. When he wore brown, people kept saying "look at that huge jawa! It speaks, too!"
    by Boinga

11. To give some of us nerds a chance to come up with silly reasons why he might wear black.
    by Jedi Knight Ivyan

12. He wanted a coat of many colors, but that was already taken.
    by Jedi Knight Ivyan

13. He's the biggest Johnny Cash fan in this or any other galaxy
    by tenacious B

14. Because that's what that "Queer-Eye Guy" Recommended for him.
    by KR

15. It goes great with his red lightsaber.
    by Kenpasck

16. Hard to stay on the Atkins diet when your only choice is the death star snack bar.
    by Rbutcher

17. Padm? was always her sexiest when dressed in black, Vader hoped for the same result.
    by Darth Nima

18. He heard that white stormtropper armor is really useless
    by Redav Htrad

19. I think falling into a molten pit may have affected his hue.
    by Rin, Destroyer of Keyboards

20. The Goth in him needed to be expressed
    by Vincent

21. It attracts the sun,absorbs heat, so it reminds him of Tatooine
    by aniluvr

22. Couldn't wear blue because he'd disappear in the bluescreen effects.
    by Darth Jester

23. Brown would get him mistaken for a Wookiee.
    by Qizara Fett

24. Just in case the message that he is evil isn't successfully conveyed by his menacing presence and cold, mechanical breathing
    by Ne Quis

25. He choked his fashion co-ordinator waaay back.
    by X-Wing Jedi

26. Hides that alderaanian ale belly and that gut from all those greasy nerfburgers....
    by Gryfft

27. Lucas to Guy In Charge of Dressing: "Come on, it's the dark side. All the bad guys gotta wear black." "No, George, that's not a good idea. White's in nowadays." "Welll, how about just one? The rest you can put in white suits."
    by Gryfft

28. Because Billy Dee Williams "called dibs" on the powder blue cape before he got there.
    by snowdog83

29. So he's easier to snipe among all those white stormtroopers...
    by -anonomous

30. he was made fun of alot by the kids in the jedi academy
    by lightningcount

31. Gray was already taken by EVERYBODY else in the Empire
    by RyanAdc

32. He needs something suitable to wear to all the funerals of all the people he kills.
    by Jake Skywalker

33. For a reason defined years ago in the Expanded Universe and generally regarded as concrete until Lucas writes the next prequel
    by Ne Quis

34. Darth: "Because George told me too..."
    by Darth Vanilla Shake

35. He'd be laughed out of the Society of Evil Overlords if he didn't!
    by Dark Helmet

36. FOOLS! Do you really believe I need a reason?! The Dark Lord of the Sith needs no reason! You all shall pay for your insolence!(reaches out with the force and asphyxiates the entire TFN staff and all who submitted a reason.)
    by Darth Vader

37. Well...originally Lucas had vader wearing a flannel cape...
    by Ladiesman

38. Because it goes so well with the interior of his Tie-Fighter.
    by Kenpasck

39. White armor caused officers to mistake him for a stormtrooper.
    by Darth Lairdman

40. What other color could disguise his shame of having fathered the most whiny brat in the galaxy?
    by Red Nine Striker

41. It has the cheapest dry cleaning bill.
    by Grim Melee

42. Because Obi Wan told him not to?
    by Darth Me

43. Got tired of looking like a damn hippie in Jedi beige and brown rags!
    by Garth Vader

44. Contrasts nicely with the Stormtroopers white armor.
    by GrrrArrrg!

45. he was told it complements his personality
    by darth_sarubious_hottie

46. u go ask him, i'll watch, from a holocamera 1 million parsecs away, on a really fast ship headed away from ur general direction
    by galahad