Black poop is often caused by taking iron tablets. However, other substances that can cause black poop are :
- Blueberries
- Black Liquorice
- Bismuth (Pepto-bismol)
- Lead
- Foods that are dark blue, black or green
As for hard poop, that is usually an indication of constipation and if you had blood mixed in your stools this would be fairly common as straining to pass stool can cause this.
However, it would not then be black, it would clearly be seen as normal poop and blood.
Hopefully, there will be a simple reason for your condition (such as a multi-vitamin containing iron) but if not, keep pressing for a medical explanation, the kind of problems mentioned above are best treated early - as delaying treatment can cause serious complications.