Originally Posted by Longblades
Blood in stool can make it look black. Though drying out can make them look black too. If you are sure they are stool and not balls, as asked above (perfectly round? You might be throwing us off with this statement) and coupled with unusual lethargy I'd Vet.
I agree with this. Black stools always makes me concerned about blood in the stool but depending upon the drying, sometimes this can be misleading.
Additionally, very hard stools, makes me want to evaluate the pet for hydration as well. If an animal has some level of dehydration present, the colon will work to remove maximum amounts of water and the result will be smaller, hard stool. With the fact that water consumption is increased, there are many conditions which will have the concurrence of dehydration as well as increased water consumption. Of course, drying and diet can make the stools hard as well.
Bottom line, I agree with a vet visit if the signs do not resolve. I hope this helps.