Bluing is an "old-fashioned" product that helps make whites appear to be whiter. Manufacturers use it in the finishing process of white goods because a blue tint makes white appear brighter. Unfortunately, the bluing wears off after repeated washings.
The good news is you can add it back by using liquid bluing. It can be used for white clothes or on blue jeans that you wish to darken. Here's how:
Always dilute bluing (it is concentrated) in cold water before adding to the washer. Never pour it directly into the machine because it can cause spotting. To whiten whites, if adding to the wash water, use 1/4 teaspoon diluted in one quart of water. Add the solution to the wash water. If adding only to the rinse cycle, use 1/8 teaspoon in a quart of water. Do not use an automatic dispenser for bluing.
For blue jeans, more can be used - up to 1 teaspoon - in the diluted solution depending upon how dark you want your jeans to be.
Bluing can be used with detergent but should not be used with bleach, fabric softener or any other laundry product.
Specific questions? Just ask here.