this will have an effect only when using simple-phpunit, not when registering the listener explicitly to be used for normal phpunit runs.

couldn't this be handled by the listener constructor instead ?

For our use cases that will work. But since the blacklist is only evaluated once I wanted to add the listener class as soon as possible. Do you think it's safe to always do that in the listener's constructor?

Well, when using the simple-phpunit runner, it would still work (as the constructor is invoked before the line you added).
For usage through the config file, I don't know. It depends when the instantiation takes place. You would need to try it. See Swiftmailer for a project using the bridge through normal PHPUnit usage and registering the listener in the config file for instance.

I can confirm that moving the modification of the blacklist to the listener's constructor still leads to the expected effect.

👍

Thank you @xabbuh.