I as well have the same problem I tried to unblock a person yesterday and today cause they sent me an invite while I was in the Rath of the Machine Raid on Destiny and than I realized I did not know that person so I blocked them only to find out three days ago it was a friend of mine who made a new account cause his got hacked so I tried all day of the 28th and 29th of october of 2016 and time kept saying "can't block any more players"
The last I remember I was trying to unblock hem not block hem twice.