Hi, I have recently done some Zulu, although only in 10mm, I think the secret to dark colours is don't higlight too much and don't be too subtle.
I used GW's scorched brown mixed with a little black as a base coat and then GW's scorched brown about 60-70% and a mid chesnut brown (by foundry) about 30-40% as top higlights. I found that when this dried a little scorched brown neat as a thin wash was enough to blend thi sin, though just how much this did I'm not sure at this scale.
I tried bestial brown as Benno suggested but felt it was a little orange when dry whereas black skin is generally a redder or even bluer cast as a base. (why scoprched brown is quite nice)
I think as with black, it you start puttign 3-4 or more shades of highlights and washes at anything under 28mm you run the risk of creating a monotone muddy mess.
Your base colour is a nice blueblack, I would maybe add a decent amount of the brown you used for the top of the quiver to the base colour (as above) a try highligh tonly the cheeks, tops of the muscles in the back and arms, calves etc. not too much. Its only something I have done a little of myself and I do perhaps want to do a zulu diorama.
Keep up the good work