Actually, I don't think it really extends battery life. Even though you have a black screen, that's just the LCD blocking the backlight. The backlight is still illuminated the same as if you had a white screen, and is therefore using the same amount of energy.
I thought I read somewhere that white on LCD screens used the least amount of energy (as small a difference as it may be) while black was most efficient on AMOLED screens (again, very small difference). There have been some discussions on some of the Android forums about it, but the feeling I got was there wasn't any earth shattering savings for either screen.
