I always assumed it was about a lover who died and the feelings of depression and despair that go along with it. He wants to "Paint everything black" to match his mood - color is usually indicative of vitality, which can be perceived as offensive or repulsive to someone dealing with deep feelings of loss.
"I see a line of cars and they're all painted black" (Hearses, presumably)
"With flowers and my love both never to come back" (Death of flowers/love)
"I could not foresee this thing happening to you" (Death is usually unexpected)
"If I look hard enough into the setting sun, my love will laugh with me before the morning comes" (Heaven reference)
I've never heard anything definitive, but those lines in particular always seemed very telling to me.