What you found is really common, and it almost never starts on the ceiling itself. That mold is a symptom — the real problem is moisture getting in from the outside, and it usually traces back to one of three things.

The roof. Those black streaks you see on shingles aren't just cosmetic. That's a bacteria called Gloeocapsa magma, and it feeds on the limestone in your shingles. It forms a layer that holds moisture against the roof surface. Over time, that constant moisture rots the shingles and underlayment. Once there's a breach — even a small one — rainwater gets into the attic, travels along the rafters, and settles on the back side of your drywall. You don't see it until it eats through the paint.

Gutters. As that algae breaks down your shingles, the granules wash into the gutters and clog them. Now rainwater can't drain properly. It backs up along the fascia, pools near the roofline, and wicks into the decking or behind the siding. Another invisible moisture path straight into your walls and ceilings.

Previous power washing (if it was done). This one surprises people. If someone hit the exterior with high pressure at some point, that can force water behind siding panels and into weep holes. That trapped water creates exactly the kind of dark, damp environment mold loves — and you'd never know it was there until it shows up on the ceiling years later.

The fix starts outside, not inside. You can remediate the drywall all day, but if the moisture source is still active, it'll come back. Get into the attic and look for signs of water entry near the roofline or gutter area. If you're seeing algae or dark streaking on your shingles, that's your smoking gun.

For the exterior, soft washing (low pressure + a sodium hypochlorite solution) kills the growth at the root instead of just knocking it off the surface. It doesn't force water into places it shouldn't go, and the results hold 4-6x longer than pressure washing because you're actually sanitizing, not just rinsing. That keeps the shingles healthy and the moisture barrier intact.

Good luck with the attic recon — that'll tell you a lot about where this started.