What is the ruling on the presence of black streaks on underwear, and are they ritually impure, necessitating a change of clothes?

The cause of the lines mentioned cannot be known; they might be from sweat or something else. If they are from sweat, the trousers are pure, and there is no need to change them because human sweat is pure. If the source of the lines is unknown, the trousers are also judged to be pure because the default state of things is purity until impurity is established with certainty.