I'm sorry to hear about your cat. What kind of canned food do you feed? Does he eat anything else (table scraps? treats?) How old is he? Has he had any bloodwork done at the vet's office? Is his urine same as always (ex/ more, less, stronger or weaker smelling, sticky, etc?)
How long has this gone on?
I would start by comparing the Friskies with other foods in the store with a higher fiber content. This on top of supplements like cottage cheese or canned pumpkin can help, as well as laxatone, which is made for hairballs but is actually an oil based product which helps move the GI tract contents along faster. The stool is staying in the intestines too long, which is why it is hardening up and turning so dark. Higher fiber is the first step. Laxatone or adding some mineral or vegetable oil to the food is another step.
There could also be an underlying reason besides just constipation, and with your cat drinking a lot of water and yet still having a dry intestinal content, it is something to consider. Things that dehydrate the body like diabetes, thyroid problems, kidney dysfunction, or other endocrine or metabolic issues can cause the body to pull more water into the cells and organs to be utilized, taking away from excess water in the intestinal tract, which softens the stool. If the above advice isn't working, I would then try having a general blood profile run by the vet to check these. Your vet could also give you prescriptions then like lactulose (another oil based product that draws water into the Gi tract) and/or cisapride, which is a drug that helps the intestines contract and push the stool out faster.
I hope your cat is back to normal soon, and wish you both the best of luck. Let me know if I can help further,