Raw cacao mass (basically melted cacao nibs) I've found to be very effective. Only downside is it can stain your teeth quite a bit, so I just grate it finely and wash it down with milk. Whenever I do this, my stubble looks denser almost immediately. I buy it from my local health foods store. It's expensive, but I feel it's worth it. Haven't tried applying it to my face directly yet, haha. Using vitamin E from wheat germ oil topically, I've noticed it makes the hair darker. Works really well for eyebrows as well.