Is faint, unclear brownish moisture on the sheet the mother sat on during her menstrual period considered vaginal discharge mixed with blood, even if it doesn't resemble blood?

If you are unsure whether the spots are impure, the default assumption is that they are pure. Do not rule them impure unless you have absolute certainty. Therefore, pay no attention to doubt. If you are certain that the spots are blood remnants but they are very slight, then they are among the impurities that are excused. One must be wary of obsessive whispers (waswas) and not give in to them.