The Collaborative International Dictionary of English v.0.48:

Blab \Blab\, v. i.
   To talk thoughtlessly or without discretion; to tattle; to
   tell tales.
   [1913 Webster]

         She must burst or blab.                  --Dryden.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Blab \Blab\, n. [OE. blabbe.]
   One who blabs; a babbler; a telltale. "Avoided as a blab."
   --Milton.
   [1913 Webster]

         For who will open himself to a blab or a babbler.
                                                  --Bacon.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Blab \Blab\ (bl[a^]b), v. t. [imp. & p. p. Blabbed (bl[a^]bd);
   p. pr. & vb. n. Blabbing.] [Cf. OE. blaberen, or Dan.
   blabbre, G. plappern, Gael. blabaran a stammerer; prob. of
   imitative origin. Cf. also Blubber, v.]
   To utter or tell unnecessarily, or in a thoughtless manner;
   to publish (secrets or trifles) without reserve or
   discretion; -- sometimes used with out. --Udall.
   [1913 Webster]

         And yonder a vile physician blabbing
         The case of his patient.                 --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

blab
    v 1: divulge confidential information or secrets; "Be careful--
         his secretary talks" [syn: spill the beans, let the cat
         out of the bag, talk, tattle, blab, peach,
         babble, sing, babble out, blab out] [ant: keep
         one's mouth shut, keep quiet, shut one's mouth]
    2: speak (about unimportant matters) rapidly and incessantly
       [syn: chatter, piffle, palaver, prate, tittle-
       tattle, twaddle, clack, maunder, prattle, blab,
       gibber, tattle, blabber, gabble]

Moby Thesaurus II by Grady Ward, 1.0:

142 Moby Thesaurus words for "blab":
   agreeable rattle, babble, babblement, babbler, bavardage,
   be indiscreet, be unguarded, bear witness against, betray,
   betray a confidence, betrayer, bibble-babble, big talker, blabber,
   blabberer, blabbermouth, blah-blah, blather, blatherer, blether,
   blethers, blow the whistle, blurt, blurt out, broadcast, cackle,
   caquet, caqueterie, chat, chatter, chatterbox, chatterer,
   chitter-chatter, clack, clatter, delator, disclose, dither,
   divulge, expose, fink, gab, gabber, gabble, gabbler, gas, gasbag,
   gibber, gibble-gabble, gibble-gabbler, give away, go on, gossip,
   great talker, guff, gush, haver, hot air, hot-air artist,
   idle chatterer, idle talk, inform, inform against, inform on,
   informer, jabber, jabberer, jaw, jay, leak, let drop, let fall,
   let slip, magpie, mere talk, moulin a paroles, narc, natter,
   noise about, nonsense talk, palaver, patter, patterer, peach,
   peacher, pour forth, prate, prater, prating, prattle, prattler,
   prittle-prattle, ramble on, rat, rattle, rattle on, reel off,
   reveal, reveal a secret, rumor, run on, sell out, sing, snitch,
   snitch on, snitcher, spill, spill the beans, spout, spout off, spy,
   squeal, squealer, stool, stool pigeon, stoolie, talebearer, talk,
   talk away, talk nonsense, talk on, talkee-talkee, tattle,
   tattle on, tattler, tattletale, tell on, tell secrets, tell tales,
   telltale, testify against, tittle-tattle, turn informer, twaddle,
   twattle, waffle, whistle-blower, windbag, windjammer, word-slinger,
   yak, yakkety-yak