dry brushing flat black?

A dark grey should do well, or I've also used a silver, washed with thinned flat black. It gives it a slightly used look.

demono69

You can also try a pale brown or light tan.

I use chrome silver base paint and a wash of thinned flat black on engine. But if you have already painted it flat black it maybe ok to drybrush it with metallic grey with chrome silver to the highest edges.

I've used #2 pencil lead to highlight the edges of flat colored objects. The graphite has a gloss appearance and will cause those edges to stand out. Also, used it on the cylinder heads on radial engines to highlight the cooling fins.

Rich

I'm going to go with Rich's suggestion. I think graphite would work very well in this instance. Not only does it highlight, but it gives a "heavy metal" look to the objects it is applied to.