I don't have the best selection of black inks since I prefer blue, but I hope this helps a bit.
I'm not sure the tints/balance came out very well, but this is the best I could tweak. The Schmidt does write bolder, although it's hard to see that here. Mostly because I find that it globs and writes unevenly. Not a fan. The ink has a bit of a metallic sheen.
The Itoya Aquaroller is a 0.7mm ballpoint, and writes like a ballpoint. Because the ink doesn't flow/bleed as easily, it's comparable to most 0.5mm gel pens. Rather metallic sheen.
The Ohto Flash Dry is a 0.5mm gel, writes smoothly, more evenly than the Easy Flow 9000. Ink has no sheen, and perhaps looks greyish in contrast to the others (doesn't really show well in my pic), but on its own looks quite black. This is what I'd recommend to you.
I don't have any, but you might want to check out the Uni Jetstream SXR-600. They come in as fine as 0.38. Can be pricey though, but all the Jetstreams I've ever tried write exceptionally well.
(pic not mine, obviously)