Anyone know anything?

Anyone know anything? I haven't been able to go to the bathroom for several days. The doctor said to put a glycerin candle in. I did, and on the next day I went to the bathroom, but now my stool is such a dark color, almost black. Could this be from the candle? I also take tablets for hemoglobin, I read that it could be from this. I will ask the doctor tomorrow at the checkup.