WordNet (r) 3.0 (2006):

unflavored
    adj 1: without flavoring added [syn: unflavored,
           unflavoured, nonflavored, nonflavoured]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "unflavored":
   dead, dilute, diluted, fade, flat, flavorless, gruelly, inane,
   indifferent, insipid, jejune, mild, milk-and-water, pappy, pulpy,
   sapless, savorless, spiceless, stale, tasteless, thin, unsavory,
   vapid, washy, watered, watered-down, watery, weak, wishy-washy