The yellowing is the result of iron/magnesium deficiency and the black is the mould caused by the aphids producing honeydew last year. You could feed in the Spring and spray with a systemic insecticide or as the shrub is still small, just wash the leaves with cotton wool dipped in a diluted washing-up liquid.

For quick magnesium, you can make a foliar spray from a solution of epsom salt.

check the stems and under the leaf for scale insect. they also secrete honeydew that then allows sooty moulds to grow.
I'd use Epsom salts in solution and water at the roots. you can buy specific feeds for camellias and rhododendrons that have the correct balance of minerals for them.

Yep, agree, check beneath the leaves for scale insect - and stems too. https://www.rhs.org.uk/advice/profile?PID=483