Short answer
Start by rubbing one of the black patches. If it smears off and the leaf underneath is green, it is sooty mold growing on the honeydew insects dropped from above, not something in the leaf. If the spots really are in the leaf, the shape is the next question: round spots that run across the veins are usually fungal, and on an established plant the published guidance treats those as an appearance problem rather than a threat, while angular spots boxed in by the veins point at bacteria instead. Feathery-edged spots on a rose, climbing from the lower leaves, are true black spot. All of them travel in water, so the change worth making is watering at the soil rather than over the top.
Listed in the order they are worth ruling out. It is normal for more than one of these to be happening at once, so work down the list rather than stopping at the first match.
First because it is the fastest of these to rule in or out, and because it is the one cause here that is not a spot at all. Sooty mold is a black coating sitting on the surface, growing on the sticky honeydew that insects feeding above have dropped onto the leaf.
What to check first
Rub one of the black patches with a damp cloth or your thumb. If it smears or lifts away and the leaf underneath is green and undamaged, this is sooty mold rather than anything growing inside the leaf.
The everyday answer, and the reason to look closely before doing anything at all. Round to irregular spots, often with a darker border and a paler center, showing up in warm humid weather after the leaves have stayed wet for a long stretch.
What to check first
Look into the middle of two or three of the larger spots in good light. Tiny dark specks sitting in the center are the fungus's own spore-producing structures, and finding them settles that this is fungal rather than bacterial or weather damage.
Worth separating from the fungal kind because the shape gives it away. Bacteria are held back by the leaf's veins, so the spots end up angular and boxed in rather than round.
What to check first
Hold a spotted leaf up to the light and look at the edges of the spots rather than their color. Sides that run straight and stop dead at a vein, instead of a round spot that runs across veins, point here.
Only worth weighing on a rose. The name gets attached to any dark spot on any plant, but the actual disease has a distinctive ragged edge and a specific habit of working its way up the plant.
What to check first
Look at the lower and inner leaves first rather than the new growth on top, then look at the edge of a single spot. A fringed, feathery margin instead of a clean round edge is the identifying detail.
Last on the list because it is the one cause here that is not an organism. Heat, wind and dry soil darken and brown leaf tissue in a way that reads convincingly as disease, and what separates it is the timing against the weather rather than anything on the leaf.
What to check first
Think back over the past week instead of looking closer at the leaf. A run of hot, windy or dry days just before the damage appeared, with the darkening worst on the most exposed leaves and along their margins, points here.
Still not sure
This is exactly what your county Extension office answers, for free. They are the local horticulture experts for your area, they know what is going around this season where you live, and they will look at a photo.
Find your local Extension officeMore than one of these is often happening at once. If the list above did not match what you are seeing, start from the symptom that fits best.
Pollinator Patch gives you watering and seasonal care for the plants you already own, written for your region rather than for gardening in general.