Incredible well known, I'd say. Pretty much all of my friends, techies and non-techies, have used it repeatedly. I'm not too surprised about this valuation.
(I checked to see if it does operate in the UK, and it seems to.)
It's used by many people in my social circle (early 20s, techies and non-techies).
So, the valuation is not so strange, except that the business model is not clear to me. In the other hand, I could say that about a lot of others startups.
So ok, I know their business model. I know ride-sharing is big in Germany (or at least used to be - not driving that much anymore).
But also I am not in their target group anymore. Would not invite strangers into my car, just to save some pennies/euros on gasoline. Nowadays my quite (or podcast) time while driving is way more valuable to me.
But if I were a student and had to drive greater distances - I would be in probably.
I believe they used to not charge a fee until about a year ago but they do now.
I hear "get an Uber" quite a lot...
edit: ok, it's more like private car sharing for longer destinations. There is a well known big website for that already for a long time ("Mitfahrzentrale") which i used a few times. But, blablacar would be only the second big offering in that sense. So there is some "space" in the market for that in Germany :)
Anecdote, I've made numerous Balblacar rides and no one has been a "techie" yet : teacher, painter, architect, hr, whatever. Mostly young people though.