Can you check beneath the leaves to see if there's anything there that shouldn't be - also the woody stems. You're looking for shield shaped objects on the stems, or fluffy white deposits under the leaves, or even small insects under the leaves.
– BambooMay 31 '20 at 10:49
It looks like sooty mould, or sooty mould previously formed and dried out a bit - sooty mould is associated with heavy aphid or scale infestation. Does the black mould wash off if you try, or is it actually in the leaf?
– BambooJun 01 '20 at 17:11
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Sooty mold is exactly what it is. Confirmed by someone who saw it live.