To blab is to gossip about or reveal a secret you promised to keep. You might accidentally blab to your dad about your brother getting in trouble at school.
A reporter could take you by surprise and get you to blab about your movie star neighbor, and it might be hard for a little kid not to blab about the birthday gift he's giving his sister. Another way to blab is simply to talk a lot: "I sat there and listened to him blab about all the famous people he knows." Blab comes from the Middle English blabbe, "one who does not control his tongue."