I would use a semi-permanant mousse or cream, one that lasts four weeks or so are usually good. My mum dyed her hair black a long time ago, just make sure that you don't lather it on other wise it makes the shade very heavy and full-on. Also, apply it in the morning if you can, my mum did it at night and went to sleep when she thought it was dry. However, she woke up next morning with a black pillow.