'Bleaches' are an interesting subject considering bleach is a word use to discribe a number of different chemicals, what we are referring to in this topic is oxidising bleaches which create oxygen within the chemical action, as opposed to reducing bleaches which reduce the oxygen, but both have the same effect which is altering the way light reflects so making it seem invisible, this is why spraying hypo on black spots makes them disappear even before you rinse
Quote from Internet..
. Natural stains (as well as dyes) produced by everything from mildew to grass come from chemical compounds called chromophores. Chromophores can absorb light at specific wavelengths and therefore cause colours. When chlorine reacts with water, it produces hydrochloric acid and atomic oxygen. The oxygen reacts easily with the chromophores to eliminate the portion of its structure that causes the colour
But bleaches are one of the safest chemicals we use