Black ink

Ink hardly dries so you'll have to spray a coat of clear varnish (gloss or matt according to the case) to fix it on the model.

Besides, black is a very dark color (actually it is not a color but absence of color), so if you want to try I suggest you thin it (usually with water) and give more coats, removing in between the excess.

add some soap to reduce surface tension and make it gradient out more evenly.  look for translucent sepia ink, it works quite well.  i forget the brand but michael's sells it as refils for an ink pen.  that's what i use for a wash on pretty much all my models.  you could probably experiment with some combination of that and the black if you'd like.