Answer: What you describe are scale insects. The adults attach themselves to a stem and feed on the fluids within the tissues of your gardenia. They exude a sticky substance called honeydew. This substance has a high enough sugar content to turn a moldy black - it is commonly called sooty mold. It can be wiped off the leaves or will weather off on its own. To eliminate the sooty mold and the honeydew, you need to eliminate the scale insects. After attaching themselves to a plant stem, the female scales give birth. The baby scales are able to crawl up or down a stem until they find a likely feeding spot. If you can catch the little scales in this crawler stage, you can eliminate them, plus any future populations so continuous spraying will be necessary until you kill off the entire population. It will be worth the effort - just keep at it!
As for hosing the pests off the plant, they usually die. If they land on grass or concrete or gravel or bark, they won't be able to attach themselves to feed and will perish in a short time. So, don't worry about them crawling back up on your plant.
Hope this answers all your questions.
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