It's sort of Periscope on steroids. With Blab you can have up to 4 people in the one conversation. A newcomer can ask to be included in a conversation by clicking on a button. There is also a text chat feature that enables people outside the conversation to engage with those who are already in it.
The problem for me is that Blab on iPhone needs iOS 8.0 and the model of phone I own only goes up to 7.1.4 or something, so the only way I can use Blab right now - until I upgrade my phone - is on the desktop. This is not so bad, for obvious reasons (I spend a lot of time at the desktop).
The one problem I have with Blab is the manners people seem to display once they are in this kind of environment, which is basically talking in a group. You get the people who just carry on regardless of the feelings of other people, for one. Then there are the people who are just really boring and who don't know how to explain what they want to say (you get lots of people using their hands to compensate for poor vocabulary and expression). And you also get dominant personalities who believe they are the main player - although with Blab this is how conversations start: you get one person inviting others to join.
So the quality of what's on off is very uneven. I didn't really find anything I would write home about during the hour I spent on Blab watching different conversations. There's a lot of mediocre stuff. Some stuff is also recorded, but the live conversations have a little 'live" icon on them. Those are the ones you might be able to join (obviously you cannot join a conversation that has already finished!).
We'll have to see how Blab goes. I expect a lot of poor productions will be enlivened occasionally by something worthwhile and valuable, but it's not a bad idea to take a look and see how it is progressing from time to time. I think this app could surprise people.