Why would you wish to turn your hair black? Grey hair is sexy. Look at all those movie stars! Well, if you really want to do then I suggest you opt for an onion paste remedy. This is fairly simply to make and use as well. cut onion into small pieces and put them in a blender along with some water. Grind them till it become a paste. You can directly apply this on the hair. Use this before you have a bath and wash it thoroughly. Continue this method for a few weeks and see what happens. Good Luck