On Mastodon, if you server block threads, you will not see any content from threads.net, including any boosts from those who are on the fedi of posts from threads.net. You will, however, see anything from instances that do federate with threads.net.
Stated another way, hypothetically, if we follow each other and you block the threads server, you will see me and I will see you but you won’t see any threads content that I boost.