Hello, Black stool is not because of betnesol..but its due to medcine which you are taking that is Iron and Folic acid table..which make stool colour black n also constipation at times..so Dont worry... Constipation is so common during pregnancy as size of uterus increases.. so it compress the intestines which leads to constipation. TO get relief from constipation.. *Take warm water.. *Take more liquids like coconut water,butter milk, milk, lemon juice, fruit juices. *+Do walking half an hour morning and evening. *Have enough rest. *Be stressfree..