Blab verb – Divulge confidential information or secrets. Let out is a synonym for blab in show topic. In some cases you can use "Let out" instead a verb "Blab", when it comes to topics like gossip, blurt out.
Let out verb – Express audibly; utter sounds (not necessarily words). Blab is a synonym for let out in reveal topic. You can use "Blab" instead a verb "Let out", if it concerns topics such as utter suddenly.