I'd say you'll need to try a bucket of bleach for a few days/nights with the carpet in, or if you're a dare devil - put them in the washing machine with bleach a few times, that should get the carpets white/lighter.
Then put the carpets in a normal wash, normally worth putting in the washing machine for a good few hours. Then once dry, put them back in, but with black dye! Might need a few washes with lots of black dye.
REMEMBER TO PUT THE WASHING MACHINE ON FOR A GOOD FEW TIMES AFTER YOU'RE FINISHED OTHERWISE EVERYTHING WASHED WILL BE BLACK!
Well - that's how I used to dye random things.
Let me know.