'darkener' originates from the verb 'darken', which comes from the Old English word 'deorcian', where 'deorc' meant 'dark'.

'deorcian' transformed into the Middle English word 'derken', and eventually became the modern English word 'darken', from which 'darkener' is derived.

Initially, it meant 'to make dark', and this meaning has largely remained the same in modern usage.