Do I see wood chip mulch? If so, it would verify my hunch that the specks are peridioles (spore packets) from one of the artillery fungi, such as bird nest fungus.

Yes, there is wood chip mulch all around here. Most of it is cedar, but there is some big-box-mystery-mulch product back there too. On plants like the lilac and forsythia, these black spots appear very high. Each of those (relatively young) shrubs is about 6' tall right now, and there are black spots pretty much right up to the uppermost leaves. Would you expect to see artillery fungus that high? And would those spores wash off? These seem brittle and hard to remove.

Unlikely to wash off.

But, even so, you can relax as they won't damage your plants.

The big "problem" with them is when they are stuck to light-colored siding or vehicles.

I would absolutely expect to see the spores that high....and higher. Just be grateful that the mulch is not used any where near your home, vehicles, outdoor furniture, etc. They stick like superglue.

As Jean says, these interesting little fungi are not harmful but are natural decomposers. Look them up and then investigate the mulch because you just might find some of the tiny little cups. They actively feed on dead wood....wood chips, old unpainted timber, and the like.

Thank you both for the responses. I'll read up more on these little fungi and give the mulch an inspection soon. Since it's harmless to the plants, I'll try not to care too much, but this area is pretty close to a neighboring home, so I want to make sure it doesn't spread. From what I've read so far, it seems moving the mulch around, replacing with fresher mulch, or using an entirely different mulch (bark or something less woody) can help keep things at bay.

I agree that using bark or pine straw can reduce the instances of the little fruiting bodies erupting all over the place.