Iíve been reading about patents darkening hair and skin at the pigmentation level. In lots of patents L-phenylalanine ( precursor of tyrosine), or tyrosine and/or L-dopa are involved
( as prescribed towards the end of the article: Chiral Skin Care :: CosMedix.com )

I think the key is to reach the follicles or dermal papilla with the right penetration enhancer.
I've been reading some topic on this over here and I guess some of you might be interested as well.
I'm not a scientist, but I'm sure some of you might be good at formulating stuff.

Some links: read 0013 Method For Increasing Hair Growth - Patent application - method for stimulating hair growth which comprises applying a composition containing at least one energy-inducing
Compositions for darkening the skin and/or hair - Patent Application 20040086471