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]