Could pentasa cause black stools ?

The drug is used for intestinal disorders associated with bleeding and that can result in black stools. The medication may itself cause bleeding as well.

Yes, as gastrointestinal bleeding is a rare, but described side effect of the drug. The condition for which you are taking Pentasa is likely to cause bleed as well, as can other unrelated conditions or drugs. Your treating gastroenterologist remains your best reference at this point.