In Peaceful mode, shriekers spawned in the Ancient Cities do not cause the blindness effect when activated. I am unsure if this is intended behaviour or not for peaceful mode, but was advised to report it here regardless. The shriekers do emit the 'shriek' noises, as well as the shrieker particles, but no blindness visuals.
Comments 2
Thank you for the clarification, I wasn't sure if that was in fact the intended behaviour so it's good to have that clarified.
Re the motivation for the report - the blindness effect was part of the general conversation during CornerHard's stream this week; he advised I report here (I am assuming from contextual clues that this was just in case it was a bug and not intended behaviour). Noted for future to add context like that when reporting issues.
The Darkness Effect comes from the Warden, not from the Shriekers.
Since it is impossible to summon hostile mobs in Peaceful mode, you won't get affected.
As a general rule, if someone advises you to report an issue, you should also say who and what the context of that was.