Thanks for posting your query.
Blackish stools would usually be with iron supplements
which is a normal thing.
If if you have started with the iron pills which you have your stool would be black due to iron.
Otherwise black stool
is not a normal thing and you need to be investigated by X-ray, Endoscopy
I suggest you to start on iron supplements only after consulting your Obstetrician.
Hope I have answered your query.
Wishing you good health.