That looks like a shield bug @Alisonjayne, a nuisance but the native species are not normally a problem. The larvae eat plant material apparently but also caterpillars. Maybe someone who knows about bugs can reassure you
"The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it." Sir Terry Pratchett
I doubt it as they are wandering sap-suckers, so unlikely to cause a lot of damage in one place. I'd gently prise those top leaves open as there might be a caterpillar inside.
A trowel in the hand is worth a thousand lost under a bush.
A great many plants host aphids, which produce a sugary fluid called honeydew. This makes the leaves sticky, and every kind of particle drifting in the air sticks to it, making it look black. Also the honeydew supports fungal growth, which discolours the leaves.
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