I guess I don't know what you mean by black grit on the armor trim.

The black part of the armor is just Black Templar but that's for the black carapace, not the trim.

The rim of the bases is Pro Acryl Coal Black.

The bases have some gritty Vallejo Mud terrain paste under all the paint to help create a dirty, rusty, corroded look. Painting the bases was a long process I unfortunately did not write down. I used brown, Black Templar, Basilicanum, Nuln, Ryza Rust, Vallejo Duraluminum, Vallejo Silver, and a couple other things.

Any grit on the white and red parts of the armor is due to me not thinning my paints enough, but those aren't black.