Beginner Gardening: Desert Rose black spots, 1 by karman

karman wrote:
We purchased this plant about two weeks ago at a nursery here in Dubai, and planted it in a pot in our yard. When we first received it it was a beautiful specimen, but shortly after, the flowers started to look kind of bedraggled, and black spots started to appear on the flowers and the leaves both. The flowers also look eaten. The black spots do come off when you rub them. The white rock below the plant looks to have dark purple spots as if juice from the plant or an insect has dropped on it. I have looked all over the plant, but cannot find any insects. Any body have any ideas?
I have attached some photo's

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