Old English bliþe "joyous, kind, cheerful, pleasant," from Proto-Germanic *blithiz "gentle, kind" (source also of Old Saxon bliði "bright, happy," Middle Dutch blide, Dutch blijde, Old Norse bliðr "mild, gentle," Old High German blidi "gay, friendly," Gothic bleiþs "kind, friendly, merciful"). Related: Blithely.
No cognates outside Germanic. "The earlier application was to the outward expression of kindly feeling, sympathy, affection to others, as in Gothic and ON.; but in OE. the word had come more usually to be applied to the external manifestation of one's own pleased or happy frame of mind, and hence even to the state itself" [OED]. Rare since 16c.