Depends on the app, some pick up and read from relays they see others mention. They write to the ones you specify but read from tons. Some apps only read from the specific relays in your list.
Discussion
i think i see what you are saying. basically any notes on the blocked relay you want to be filtered out of the collated feed of notes from all other relays. i guess it could be done by a client but it seems like it would be a rarely used feature
ah yeah I see, interesting problem. potentially lists can solve for this? There’s already the NIP for muting pubkeys, maybe this could be extended to relays