For those affected by the blocks, Signal recommends turning on its censorship circumvention feature.
Signal is being blocked in Venezuela and Russia. The app is a popular choice for encrypted messaging and people trying to avoid government censorship, and the blocks appear to be part of a crackdown on internal dissent in both countries.
I think you are answering your question yourself. There’s nothing you cannot fully block, unless it uses vpns/proxies and then you can try and block those too.