If those canned foods are darker meat, the poop will be dark too. I've fed venison, they are darker than chicken meat (like beef), but I also feed raw. I feel like canned foods usually ends up as dark brown, though.
You could also take in fresh stool samples to the vet and see if they can find out whether or not your cat does indeed have blood in her stool. Just because you can't see blood, it doesn't necessarily mean it's not present. If there happens to be an upper GI bleed, feces tends to be black. If the problem is in the lower GI tract (large intestine and near the rectum), the more likely you'll see the red. Hope it's not the case, but doesn't hurt to bring in a fresh sample to get the worry out of your mind.