A couple of leaves are displaying these colours and while it looks OK I get the feeling they should be like this. An idea on what might be happening?
Hydrangea leaves turn several shades of reds, pinks and purples in the Fall just as the plant begins to go dormant so the color would not concern me. The darker or black spots look like hydrangea leaf spots or some other type of fungal infection. If this is Fall time where you live, I would assume that you have a fungal infection and use good sanitation techniques. Yes, you could apply a fungicide but (1) some fungicides can be expensive, (2) it is late in the growing season so your leaves will be drying out soon and (3) the fungicide needs to be the correct fungicide for that fungal infection. Thus, sometimes it does not pay to apply fungicides this late.
I would assume that other leaves will start displaying similar symptoms as they too begin to dry out and go dormant. To prevent the spread of the infection, take measures that will prevent the spread and minimize the conditions that created the problem. Some suggestions:
* never water the leaves (that is, no overhead watering)
* water the soil instead but, always water very early in the mornings
* reduce humidity levels -if you can- by either improving air flow between this plant and other plants nearby, watering less or somehow allowing more sunlight in the area so the plant gets an extra hour or so more of light
* badly infected leaves should not be allowed to fall on the ground if you can. They should not be added to the compost pile either; instead, dispose of them in the trash. Dispose of the blooms in the trash too (when they dry out).
* replace your existing mulch with new mulch as soon as the plant has gone dormant (the new mulch will not have as many fungal spores as the one you have now has)
* maintain the area below the plant clear of plant debris (leaves, sticks, etc). Any debris should be thrown in the trash so you do not spread the fungal spores.
* you can send a leaf in a sealed transparent envelope for analysis from your Agriculture Extension Service so you can begin a fungicide program early in the Spring.
Frequent rains intensify leaf spots but lack of rain suppresses the symptoms. The damage can be difficult to see at first but it becomes more obvious as leaves begin to dry out in the Fall and the green pigment in chlorophyll does not hide them as well as it used to. By the end of the growing season, lesions that the leaves may have suffered will show up like this as the leaf spot fungus takes advantage of little cuts to affect the plant.
The disease is not fatal but it just makes the leaves look ugly. Some solutions used to treat the problem: neem oil, baking soda spray solution (combine 1 tablespoon of baking soda, 1 tbsp of horticultural oil and 1 gallon of water and spray the top/bottom of leaves plus the stems) or a fungicide whose active ingredient is chlorothalonil.
Does that help you GrantCC?
Here is a link that might be useful: Some leaf diseases of hydrangea courtesy of ACES
Many thanks for the comprehensive response Luis. In fact it's early summer here and these hydrangeas are underneath some tree ferns so it is definitely humid, but I'll try what you've suggested and see how we go.