The Collaborative International Dictionary of English v.0.48:

Blithe \Blithe\ (bl[imac][th]), a. [AS. bl[imac][eth]e blithe,
   kind; akin to Goth. blei[thorn]s kind, Icel. bl[imac][eth]r
   mild, gentle, Dan. & Sw. blid gentle, D. blijd blithe, OHG.
   bl[imac]di kind, blithe.]
   Gay; merry; sprightly; joyous; glad; cheerful; as, a blithe
   spirit.
   [1913 Webster]

         The blithe sounds of festal music.       --Prescott.
   [1913 Webster]

         A daughter fair,
         So buxom, blithe, and debonair.          --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

blithe
    adj 1: lacking or showing a lack of due concern; "spoke with
           blithe ignorance of the true situation"
    2: carefree and happy and lighthearted; "was loved for her
       blithe spirit"; "a merry blithesome nature"; "her
       lighthearted nature"; "trilling songs with a lightsome heart"
       [syn: blithe, blithesome, lighthearted, lightsome,
       light-hearted]

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "blithe":
   beaming, blase, blissful, blithesome, boon, bright,
   bright and sunny, carefree, careless, casual, cheerful, cheery,
   chirk, chirpy, chirrupy, delighted, detached, elated, eupeptic,
   euphoric, exalted, exhilarated, flushed, gay, genial, glad,
   gladsome, gleeful, glowing, happy, happy-go-lucky, heedless, high,
   hopeful, in good spirits, in high spirits, indifferent, insouciant,
   irrepressible, jocund, jolly, jovial, joyful, joyous, jubilant,
   laughing, light-hearted, lighthearted, lightsome, merry, mirthful,
   of good cheer, optimistic, pleasant, radiant, riant, rosy,
   sanguine, sanguineous, smiling, sparkling, sunny, uncaring,
   unconcerned, winsome