BlaBla car experiences ?

Since Blabla is used (I believe) for electronically-assisted hitchhiking (or at least for side arrangements on car transport), I would think that the individual driver is the key.

I considered it in France but I couldn't get my phone to find an app.

Used it several times in Russia (Moscow to Saransk, Nizhny Novgorod to Crimea).
No hickups, but I probably lucked out with individual drivers.
Given that more often than not the riders are supposed to keep the drivers awake over a long journey, I'd think lack of language barrier is key.
It's a viable option, but I would keep it in mind as a last resort.

I have used it both for France and Spain and it worked really well.
My son did Tarifa to Seville, no problems.
You can choose the driver based on the reviews.