sounds like sooty mold due to aphids. Some of mine get this as well but I'm not sure how to prevent it.

Sooty mold can often be successfully removed by rinsing with water from a garden hose. However, this will be most effective after the aphids or other honeydew-producing insects are destroyed. Sometimes sooty mold development is so severe and persistent that only time and exchange of leaves through the growing seasons will remove it all.

Aphids are not especially difficult to control and non-chemical methods are usually very effective. Insecticidal soaps are often the remedy of choice for many gardeners. Various brands of insecticidal soaps may be found at just about any retail nursery or garden center. Be sure to read and carefully follow all label directions when using any pesticide, organic or chemical.

Buy some ladybugs. Aphids are the favorite food of the ladybug. Water first before letting them go so they have something to drink with all the aphid eating that will take place.

Besides the soaping, give 'em Medina. It's like chicken soup for plants and my crepes seem to respond real well to the stuff. Your crepes can outgrow the malady if you feed them.

Are they in a sunny location with good drainage.

Some of those kind of problems develop when they are weakened by a location they don't like.