Is dark staining in ceiling insulation mold or just soot?

Those dark spots are almost certainly just heavy particulate dirt and accumulated soot rather than mold. You can tell because mold needs consistent moisture to take hold and grow, and you’ve checked the drywall and joists for any water stains or signs of dampness. Since you're seeing the spots specifically near a wire entry point and mentioning dust around that same area, it suggests the material has simply been drawn in through air leaks over time. Insulation, expecially fiberglass, is porous, meaning it's like a giant filter for the air in your utility room or attic. If you have drafts around penetrations for plumbing or electrical wires, fine dust, soot from the surrounding structure, and dirt from outside can easily travel and settle deep within the batts. The black and gray coloring is classic for urban dust and ambient grime. Even if you think the area is clean now, microscopic organic matter can survive for years, which is why old water damage concerns can be tricky, but without visible fuzz or a green or vivid black color, mold isn't the primary concern. To be safe and get rid of any potential issues, you should thoroughly clean that section. A low-pressure vacuum with a HEPA filter works well for pulling the loose particulate out. Then, if you want to neutralize any possibility of spores, a simple dilution of hydrogen peroxide is effective. Just spray it on and let it air dry for several days before any cleanup. Because mold thrives on organic material, even if this is just dirt, cleaning it out prevents future problems if conditions change.

Once the area is completely dry and clean, focus on sealing those entry points. Addressing the air leaks around the wires and framing will stop the infiltration of new dirt and spores. If you are replacing the insulation in that spot, expanding foam is a good way to seal the gaps around the wiring and framing to prevent future drafts.

I saw someone talking about the dust getting into the batts around the wires. My thinking is that if it is just dirt, maybe the heat in the attic could dry it out eventually, even if it looks gross. But since the utility room gets pretty humid in the winter, I'm worried that moisture might just get trapped in the fibers over time. If I vacuum it out now, how long til any new dust settles back in?

I think the concern about what happens after you clean it is entirely valid, because when you vacuum out the visible particulate matter, the underlying fibers of the fiberglass still retain a degree of porosity. If the local relative humidity is high and the space remains unconditioned, micro-deposits of airborne grime or even surface dust could theoretically re-adhere to the material over time, because the static electrical charges on the insulation itself might attract these smaller particles.

That said, since the bats are designed to be relatively dry, the chance of organic growth becoming established again is low unless you are introducing active liquid moisture, which is a much higher threshold than just atmospheric settling.

I've had stuff like this in my own attic, and it's always just grime. Once you vacuum all the loose fibers out, you really should just seal up the gaps around those wires. If you seal it properly, you shouldn't have to worry about it settling back in for ages. The real issue is just finding those little air leaks, which ussually take a bit of patience to track down. Sometimes you just gotta use a smoke pencil to find where the cold air is coming through.

I always treat these dark spots with extreme skepticism, becasue while dust is common, sometimes you're dealing with something that suggests a bigger issue. When I pulled insulation like that out of a guest room a few years back, the grime was mixed with what looked like old insect casings and wood dust. It wasn't just fine air particles settling. It made me wonder if you checked the wood structure directly above that spot. If those batts are acting like a sponge for decay, you might need to look at the headers or framing above the cavity, because the dirt might be coming from the structure itself. That changes the whole repair job.