How do you mix hair colors?

How do you mix hair colors?I began dying my hair black in February.I'm a natural brunette.I got sick of my brown hair.I like black hair.So I have been dying it black.I have been experimenting with the different shades of black.I am going to try blue-black for the first time at the end of this month.I decided that when I get middle aged and elderly that I'm going to dye my hair blue-black.I want to see how it looks now. I want to try mixing black dye with different colors. I want to try the plum pion purple dye but I want to mix it with a few drops of black to make plum-black. How many drops of black dye to add to the plum hair color to make plum black? What about burgundy black?How many drops of black to add to the burgundy to make burgundy-black? I just want to experiment with different shades of black.