You need not worry about this for now.
Frequency of motions increases as baby grows in size, and the uterus
size created pressure on the stool collecting pouch in the body. This would stabilize in a few weeks.
The black colour of the stool is beacuase you are taking iron tablets, which is a common cause for black stools and is not dangerous , so you should continue with the Iron and calcium tablets.
However, you should keep monitoring yourself for any bleeding (blood clots
or red blood ) in your stool and if this is the case you should immediately visit your OBG/ Physician for understanding it.
Wish you a wonderful motherhood!