This is from hearsay, but from what I understand from the (french) site you are paying Blablacar when reserving, and then you give a code to the driver IRL before the ride, so that he/she can redeem the payment. So you don't pay "directly" the driver.

Here is a link to the UK site : https://www.blablacar.co.uk/faq/question/how-are-the-booking...

So the booking fee perceived by Blablacar is about 10% of the cost of the ride.

It is possible you used an earlier iteration or that different market have different prices. It is very possible booking is free in new markets.