It's probably a display bug. Earlier my Peacemaker was doing what seem to be normal damage ( 55s) to gun puppies, however, i did another run and it displayed grey numbers while doing the same damage.

I am almost at level 5 on my Antiqua and will be crafting the Blackhawk so I will let you know in a little bit

Dual damage-types makes your damage more "balanced";
super-effective hits won't score as much bonus damage, but resisted hits won't be reduced as much.

Antigua lines in general deal relatively low damage for guns anyway, so the dual-typing with damage scaling won't make it strong enough to see "crits" until the latter of Tier 3.

The reason you notice most your numbers being gray is because of the dual damage type of the Blackhawk. Blackhawk = piercing and shadow. Any enemy that has high defense vs any of these means gray damage. Even if an enemy is weak to piercing but strong to shadow, or vice versa, will still be gray because damage penalties overshadow damage bonuses by a very large margin.

Using a Faust in the Royal Palace against the mini jellies, I would get critical coloring on the first hit and normal coloring on the second hit. I tried many times to get screenshots of it, but really hard to time it right. The first hit was doing less damage than the second hit even, but it was the one that showed up as critical. The mini jellies aren't even supposed to be very weak to shadow like the normal jellies.

So I think that the formula used to determine the coloring is complicated. Might be some mix of hit magnitude and damage sensitivity. Once the result is over some threshold it'll turn critical colored. Which means that you might be doing "critical-level" damage but the color just doesn't change because the ratio of damage types isn't quite right to trigger the effect.

This has happened a number of times with other monsters and weapons but I don't think I've ever seen this problem with single damage type weapons like the flourish or the haze bomb line.

Mobility and whatnot is unrelated though, of course.

Anyway, that's why I said they're collective observations; they're based on what several other players, friends and I have noticed throughout our careers (lol). On the flipside though, I haven't used any other guns since the official release, so I'm not keen on the damage now.

And yes, I did mention that resisted hits won't be as reduced.