Past tense of dark used as a verb; became dark or made something dark (archaic or literary usage).
From dark (Old English 'deorc,' meaning absence of light) used as a verb + -ed (past tense suffix). This is an archaic verbal form that's rarely used in modern English.
Most people think 'dark' is only an adjective, but in older English texts you'll find 'the sky darked' meaning 'the sky became dark'—it's a reminder that word categories shift over time.
Complete word intelligence in one call. Free tier — 50 lookups/day.