From The Collaborative International Dictionary of English v.0.48 :
blabby \blab"by\ adj.
same as blabbermouthed 1.
Syn: bigmouthed, blabbermouthed, talkative.
[WordNet 1.5]
From WordNet (r) 3.0 (2006) :
blabby
adj 1: unwisely talking too much [syn: bigmouthed,
blabbermouthed, blabby, talkative]