I don't know how you can naturally darken your beard and promote growth. If such a thing existed I'm sure everyone would be well aware.

For myself, not sure what it is but any conditioner or shampoo I've ever tried on my beard just ruins it and it takes a long time to recover and for the hair to look and feel healthy again. I have been applying oil before a shower and letting it soak in for an hour or so seems to help a lot and gives the beard a great shine making it look a bit fuller and bigger. I used jojoba oil for about a month and now experimenting with grape seed extract oil. It's much thinner, really doesn't have any kind of scent is is about 1/4 of the cost.