Usually when poop is dark and in balls, it's moving slow through the system and without much liquid (this is same for us as well). The faster the poop moves, the lighter it is and the wetter it is.

I suggest getting some proper canned food for cats as it has a high liquid content, and supplement him with that. Dry food is just that...very dry, and cats may not always drink enough water to help it move through easily.

With regards to blood, if you see fresh blood, or if his poop is black and tarry, then it would be worth a trip to the vet.

However, if you are concerned, it never hurt to contact the vet and ask them, especially if it will bring you peace to know what the pros have to say.