I can see what look like small black lumps in amongst the sooty deposits in your picture. Check the stems and branches and backs of leaves for infestation such as scale insect, but otherwise, is this plant underneath a tree?

Check the undersides of the leaves and look for small pale green 'discs' against the midribs. If they're visible, then it's scale insect, curiously more often a problem on pot grown plants, but a particular pest on plants like camellia. However, they wipe off easily enough, as does the black stuff, sooty mould, which forms on the 'honey dew' the scale insects exude as a waste product. You can spray, but I prefer not to. As Bb asks, is the holly under other trees? In summer, many trees are host to aphids of all kinds, all of which drop sugary waste onto thngs below, including cars. Sycamore and lime trees are great ones to park under if you want a black mouldy film on the metalwork! But at this time of year I expect the infestation is actually on the holly. Worthy