When you receive a downvote your reputation level drops. If it drops below a certain point then your posts and comments might get hidden. But hidden or not depends on the frontend, it might be hidden on hive.blog but not hidden on another frontend. Someone else can come behind and upvote the hidden post or comment, with each upvote the reputation level will go up, if it goes up high enough, the post or comment will be shown again.
The blacklist is specific to each user. I can have my own blacklist, you can have your own that is different to mine. Someone else can decide to follow your or my blacklist.
Thanks for your replies .
I want to know more info, what is diffrence between hive watcher's blacklist or normal user blacklist ?
If I blacklist any user, then what it's loss to that user ?
You have more info on Hive's blacklist here:
https://hive.blog/hive-102930/@blocktrades/hive-improvement-proposal-decentralize-blacklists-on-hive
There is no difference between Hive Watcher's blacklist and a normal user blacklist. But Hive Watcher might use their own blacklist not the Hive blacklist.
If you blacklist any user, there is no loss to that user. What happens is next time you see that user's posts or comment you will see a red number next to their name to remind you that you blacklisted them or they are blacklisted by someone you have followed the blacklist. For example, I've done a little demo, I added @steemulant to my blacklist for test and when I go to one of its posts, I see the red number 1 telling it's in my blacklist.
The Hive blacklist is just for you to maintain your own blacklist. Instead of having your own blacklist, you can also follow someone else's blacklist. For example if you trust Hive Watchers, you can follow their blacklist and you will know who is blacklisted by them. This is useful because maybe someone you don't know is a spammer or a phisher trying to steal your password, if you see that red number near their username, then you know they were blacklisted and you will be more careful.