My dog has been having jet black poo's which are extremely drie and it crumblies in your hands but he is still himself in all aspects what is wrong with him

Sounds mostly like he is not getting enough water and maybe change his eating habits some if giving him more water doesn't help.

Your dog may be extremely dehydrated and not getting enough nutrients. Switch his/her food to ORGANIC dry food. If you can afford raw organic dog food, try that. But it does come at a cost. Most pet food stores sell vitamins, I recommend something with an amino acid in it, or something with omega-3s/fish oil. And tons of water! Even if you must stand next to your dog and make him/her drink, do it do it do it!:)

Crumbly pooh is a sign of dehydration. Check to see if his gums are dry and sticky (dehydration) or nice and moist. Black stool is one of the main signs of internal bleeding from the GI tract. So you have a possibly dehydrated dog what may very well have internal bleeding. He could very well not be displaying other signs of illness yet. I'd definitely take it to the vet and would do it right away.