Do you take Pepto-Bismol? This can change the color of stool. I found this out when I visited my Dr. because I did have a dark stool. It has since changed back to somewhat normal.
although, iron tablets can make the stool black, sometimes. So this might be considered if you are taking iron tablets.
If they are truly black, and not just really dark, then this needs to be checked out by the dr. This is indicitive of "old blood" in the stool.