It's called plugins, and it's great. Technically you are breaking ToS when using them, but there hasn't been any crackdown (the betterdiscord team did remove the more 'risky' mods like messagelogger from the plugin store)
I don't even mind the 1 ignored message. At least it gives me slight context that people are replying to something. I hate that it gives me the little "new messages" indicator on chats when it's only users I have ignored.
Well, you still see the blocked message, just not the contents of that message. They can also still see your messages, they just can't react with emotes. This is just feedback I've seen from people who use the feature, as some people (not necessarily me) take issue with these things
It was fairly easy for someone to know you blocked them since they wouldn't be able to react to your messages or suddenly couldn't dm you anymore, I assume that won't apply to this.
the blocking feature would let the blocked person know when they try to DM the user or react to the user. this "ignore" feature appears to hide all messages and DMs while still allowing reactions