Uses
The major use is in changing the color of steel, especially the steel in guns, the so-called "blueing" process which uses selenous acid, copper(II) nitrate, and nitric acid to change the color of the steel from silver-grey to blue-grey. Some older razor blades were also made of blued steel.
Another use for selenious acid is the chemical darkening and patination of copper, brass and bronze, producing a rich dark brown color that can be further enhanced with mechanical abrasion.
Selenious acid is a key component of the Mecke reagent used for drug testing.
Read more about this topic: Selenous Acid