I use very thinned down black paint(not ink!, because that shines), wich wil leak down into the crevices. But this doesn\'t mean you can get al brush-happy with it, because it has to remain in the crevices that need blacklining. Wich is mostely between two different colors or objects(wich aren\'t in one piece with each other) eg. skin->cloth or bag->random object).
But you have got also darklining(wich I prefer to defina as different than
blacklining), this is when two parts of the same object/shap overlap each other. Then a dark crevice fill up will do(darker than the main near darkest color).
Eg. you have an arm on an other arm(or piece of body, whatever) then crevices where the two parts edge are to be filled up with a dark shade of the skin(like, if you have human skin using browns for base, then a thin wash of bestial+schorched brown(GW) or so will suffice)
Hope this helps
(my appologies for any not-understandable english, I\'m dutch afterall
)