Usually when you type in a username to add them, it is never because they blocked you, but rather, as stated above:
1. The user's account was deleted.
2. The username was typed incorrectly.
Considering you stated the username was typed correctly, and given that you are trying to add an old player, I can only assume that the account in question was deleted (likely because the account may have been inactive for a year or more, and was automatically removed by the system), so you unfortunately won't be able to add them considering their account is likely gone.
I hope this helps, and thank you to everyone for helping as well!