White streaks on black jeans can result from the use of too much washing powder, undissolved detergent or hard water mineral deposits. Additionally, the fade-resistance of the dye used to colour the jeans can affect whether the jeans eventually start to streak, as some dyes fade more than others. Getting the white streaks out of your black jeans requires that you determine the source of the streaks and then treat/repair the jeans as needed.
Check the creases of your jeans for powder, or scratch one of the white streaks on your black jeans with a fingernail.
Rinse your jeans if you found obvious signs of laundry powder or mineral deposits or if you were able to scratch away some of a streak with your nail. After you rinse your jeans, turn them inside out and dry on low heat, or line-dry, to prevent fading.
Dye the jeans if you suspect that the streaks came from fading. The exact method to dye depends entirely on the jeans-approved black dye product that you purchase. Typically, this involves soaking the jeans in a sink or bucket filled with a solution of water and dye.
Hire a water specialist to add water softeners or add 1/2 tsp of salt to wash water before washing your jeans to help dissolve hard water deposits.
Turn the jeans inside out before washing to prevent the dark exterior of your jeans from rubbing and wearing when swishing around in the washer.
When using powdered washing powder, pour it into your wash water and wait for it to dissolve completely before throwing in your jeans to help prevent streaking.