Can menstruation return after it has ceased and the woman has purified herself from it, knowing that the recurring blood consists of internal drops and is black in color?

If the six days are your wife's most common days of menstruation, and she becomes pure after them, then the blood she sees after that is not considered menstruation, because it appeared after her usual days. It is known that the minimum period of purity between two menstruations is 15 days according to the majority of scholars.

If the blood returns during her usual period, it is menstruation. However, if it returns outside her usual period, it is not considered menstruation unless it reoccurs three times, provided that the total of the first and second bleeding and the period of purity between them does not exceed fifteen days.

However, if her usual period exceeds these days, then the blood she saw after becoming pure is considered menstruation according to most jurists.