why are my aucuba japonica leaves turning black

is it just an odd leaf or the whole plant? an odd leaf is just normal death of a leaf and nothing to worry about. if its the whole plant then i suspect there is serious root damage. Knock it out of the pot and see if there are any grubs in the compost.
or it may be due to drought.
They dont usually like full sun either.
sorry i cant give a definitve answer but some-one else might have a better suggestion.
welcome to GOY too.

aucuba like semi-sahe to ful shade with dry to some wet at root, but not like sit in wet erth.