According to its website, Blab.im is still in its beta phase and is a developed by Bebo. It is a livestreaming platform that enables a public video chat among four participants at a time.
From a review on the C4compete blog:
"If you are familiar with the likes of Periscope, Meerkat and Google Hangouts, it basically combines all three. Viewers come in to watch the livestream and they can participate by leaving comments on the left hand side of the desktop application or below on the mobile version. If one of the four seats is open, a viewer can easily request to join and it’s up to the host to accept them into the livestream."
The Blab.im website is cryptic, more about the story than the tool. So the best way to find out how this tool works is to download it and start hosting conversations.