Taking black seeds or any pills to induce menstruation cycle to start may disturb and change period or have negative effects on health, especially if it is taken continuously without consulting a physician. This is not recommended.
It is on this that the prominent Muslim scholar and Da`iyah, Zeinab Mostafa, states:
Almighty Allah created every woman with a special system that varies from one woman to another. Therefore, there is no need to take anything that disturbs this natural cycle that Allah created you with. Let it go on its natural course.
However, if you took the black seed seeking the blessing that the Prophet (peace and blessings be upon him) spoke about, then that is fine, but I do not recommend you take or consume it without a doctor’s advice.
Taking herbs excessively can have negative results, so we need to seek medical advice. Also, if you are worried about the time your monthly cycle should start, you may also consult your family doctor.