okay. so i finally have an answer to this one, for i have two viber accounts, and i blocked myself, okay. and i also had 2 people block me on purpose just so we could compare.
i think viber made some changes over the years, and right now, as long as the other person's viber account is in the default mode (he never tinkered with the privacy setting), this is how you should know if you were blocked, without having to message him/her or any indiscreet method others are proposing. Really guys, sometimes, a person's just got to know if he's blocked or whatever without looking like he's still trying to get in touch. anyways... because i have so much time on my hands and i dont wanna take Viber's statement for it...
For the latest Viber version update 7.6.0 (year 2017 version)
Given that this person allows his/her last seen status and profile picture visible to all by default, immediately after blocking, the first to disappear is the Last Seen status. And then after 2 hours or even less, the picture will also disappear. Also, the message you send will not show any delivered/seen status at all. if you try to add this person to a group chat, viber will not also allow you to do so.
meanwhile, if the person is a part of a group chat you were in, you will still see his messages, and picture only in your group's chat box. (but his profile outside of that group chat will still be empty, so that difference makes it pretty obvious )
i dont really advise for you to send a message because hello, have some self respect, if the probable reason was that maybe you felt the other person was already avoiding you, why else would you try to send a message, or even try adding this person to a group chat? because if the person has not blocked you YET, Viber will definitely let him/her know about that activity and that would be super awkward! maybe you'd give him a reason to actually block you. so stop being annoying.
Now, since there is a privacy option for some people to not allow his/her last status seen, and there are people who take down their photo, you need to get the help of other people, or have a new viber account to confirm what has just happened. simply sign in to another person's viber, then add the number of the person whom you think blocked you, and voila.
1. if the profile shows both the photo and last seen status, then yes, 100%. you had been blocked.
2. if the photo is not there but last seen is present, you had also been blocked. it just happened that the person had his settings to not allow anonymous people(not listed in his contacts) to view his picture.
3. if the photo and last seen status arent visible, same as what appears in your own viber account, then have some hope, that well, maybe, just maybe, you guys are still okay. but that does not guarantee that you werent blocked. it is actually very hard to tell. nonetheless, move on with your life.