Hi!!! If they have become darker but you don't have fungus you may try lemon juice. It helps a lot and if you mix it with bicarbonate it is even better... hope this helps and good luck =)
It sounds like you have toenail fungus. You need to take an antifungal medicine orally (Lamisil) for 3 months and you will lose the nail(s). The natural remedies don't work to my knowledge as you have penetrate the nail and the other drugs don't do that.