I recently dye a black pants with fabric pant. After I put out to dry for 72 hours like the bottle said. It left some off white or gray resdue

I soak them in Oxiclean to try to remove the resdue leave behind.
Now the water is all black. I also read if you put bluring in wash water it restore black to orignal colorhttp://cleaning.lifetips.com/cat/7405/stain-removal-from-clothes/index.html

Here small article:

Refresh Black Clothing
You can refresh your black clothes by adding bluing, or strong coffee, or tea (2 cups) to the rinse water. They should return to their original dark black state. To prevent future fading, wash them in cold water, with Ivory Flakes plus only a small amount of detergent.