Ok, thank you for this information.
It may simply be that the poo has less moisture content than before so appears darker. This can happen if a dog is drinking less, exercising more or if the weather is warm/humid.
As he is quite well and with no other symptoms (such as weight loss, vomiting, diarrhoea) there is less chance it is something sinister.
While darker stool can sometimes indicate digested blood in the stool, we would normally have other symptoms such as diarrhoea, vomiting, pale gums, lethargy or reduced appetite.
Royal Canin is a good quality food and I would continue with it.
I would ensure he is up to date with a good quality wormer such as Panacur or Drontal.
I would see if increasing the amount of water he gets helps. This may mean adding water to his dinner or encouraging drinking by offering e.g. the water that chicken is boiled in.
If the problem continues or if he develops other symptoms, the next step would be a couple of simple diagnostic tests to ensure there is nothing more worrying going on. This would normally mean a simple blood test to ensure he is not anaemic and a faecal exam.
In an older dog, we may also consider abdominal imaging (especially if there is evidence of anaemia).
It's also sensible to ensure there are no potential toxins about he may be getting to, such as rat bait in the garden or human medication within the home.
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