But you can test what the gas pedal does pretty easily. I think parent's point was that without peer review source code audit, we just have to take it on trust.
The analogy doesn't hold because gas pedal's tend to do only one thing. A binary blob can do anything. A better analogy would be what some random wire in your car does... but even then if you know where the wire is located you can reason what it probably does. In this case a binary blob can't as easily be inferred.
Is there a difference between knowing what pressing a gas pedal in a car does and how it does that? Surely there's none.