What speakers mean by it now
Ask for the origin of the word bleach and most people point at the bottle: a chlorine or peroxide solution used to whiten cloth, kill germs, or lighten hair. The verb covers the same ground. You bleach a shirt in the wash, the sun bleaches a curtain, a stylist bleaches hair. Biologists extended the intransitive use to coral, which turns white when heat or other stress drives out the symbiotic algae that give the reef its color. Figurative English can bleach a word of its meaning, leaving a grammatical husk.
That cluster of senses is familiar. Only part of it is old. The chemical product, and the habit of treating the noun as the name of a household reagent, belong to industrial modernity. The verb is native, Germanic, and already in use in Old English for making cloth pale.
A verb English never needed to borrow
Middle English blechen continues Old English blæcan. Historical dictionaries place the verb before the twelfth century, which is to say in the Old English period, not among the French loans that arrived after the Conquest. The early uses sit on fabric: to make a textile white by taking color out of it, whether the agent was sunlight, air, or some wash the laundry knew.
The word did not travel into English from Latin or French. It is inherited from West Germanic, and its closest relatives are still transparent. Dutch bleken, German bleichen, Old Norse bleikja, Danish blege, and Swedish bleka all mean to whiten or make pale. Beside those verbs stand the matching adjectives: German bleich, Dutch bleek, Old Norse bleikr, all pale. English kept a near sibling in bleak, and an older pale adjective blake that has largely left ordinary speech. The immediate path is the same in the standard historical dictionaries: Middle English from Old English, related to Old English blac, pale.
A causative built on pale
Germanic reconstruction gives a weak verb *blaikijaną, to make white or pale. Morphologically it is a causative: the suffix that derives verbs of causing, attached either to the adjective *blaikaz (pale, shining) or to the related strong verb *blikaną (to shine). Those two analyses are not rivals so much as two ways of cutting the same root. Old English blac already meant bright, glittering, and pale at once, so the derived verb is, in structure, to render something that color of light.
Behind the Germanic forms lies an Indo-European root of shining, flashing, and burning, commonly cited as *bhel- and, in a more specific shape, dialectal *bhleig-. That narrower form is compared with Old Church Slavic verbs for sparkle and glitter, a Slavic noun for a lightning-flash, and Lithuanian verbs meaning to shine and to turn white or pale.
The same family is widely, if cautiously, linked with black. The link is treated as probable, with two motives and no need to choose between them: black and white can both be thought of as lacking hue, or both can be thought of as products of fire, brightness on one side and char on the other. The caution is warranted. In Middle English the spellings blac, blak, and blake are often hard to read as dark or as wan, and the surname Blake has been explained as naming a pale person or a dark one. Old English even had a verb, scimian, that could mean to shine and to grow dusky. Opposite color senses from one root are not impossible in this family; they are also not a story that can be tidied into a single cause.
Cloth on the hedge, then chemistry in a bottle
For most of its life the English verb stayed with textiles. Cloth was washed and laid out; later, industrial chemistry brought chlorine and sulphurous acid into the works, but the word itself did not have to be reinvented for the new agents. An intransitive sense, to become white rather than to be made white, is in place from the early seventeenth century. That is the grammar of bones bleaching in the sun, and of linen left on a hedge to whiten, without a named worker doing the work.
English once had nouns in this family that did not become the modern label on the bottle. Old English blæce named a skin complaint then associated with leprosy. A late Old English blæco meant paleness. Middle English blech could mean a whitening agent. Those forms receded. An archaic adjective bleach, pale or bleak, is a thin survival of the old pale sense. An obsolete medical noun for a hypopigmented, itching skin disease is a fossil of the Old English complaint-word.
The modern noun is a fresh derivation from the verb, not a straight continuation of those older substantives. Sources do not agree on the year. One standard dictionary dates the noun, in the sense of the act or process, to the mid-eighteenth century. Other historical notes put a bleaching-agent sense in 1881 and an act-of-bleaching sense in 1882, and treat the current words as independent late-nineteenth-century formations. The discrepancy is real: process and product need not have been lexicalized together, and the chemical-agent reading is clearly late. What is not in doubt is the direction of travel. The bottle takes its name from the old verb. The verb does not take its name from the bottle.
Later English added specialized intransitives without disturbing that core. Coral bleaching is a twentieth-century biological application of the same fade-to-white idea. Semantic bleaching, in linguistics, treats meaning the way the laundry treated dye.
The seats that remember the field
A concrete relic of the older trade sits in American stadiums. Uncovered baseball benches were called bleaching boards in the late nineteenth century because they resembled the frames on which cloth was spread to whiten in the sun, and because the boards themselves went pale in the weather. The Cincinnati press is already using that image in the 1870s. From the boards came bleachers for the seats, and then for the people who sat in them. The suffix is ordinary; the image is not. It preserves a time when bleaching was work done with light on a frame, not a measured pour from a jug.
Dutch and German speakers can still hear the pale adjective inside their verbs. English speakers mostly cannot, unless they notice bleak, or the long gap between blæcan on cloth and bleach in the cupboard. The verb is the old word. The household noun is the recent one.