The block list has a maximum limit of 50 blocked accounts. Once you’re at this limit, and you try to block another one, you get the failure message. In-game, from the Menu choose Friends then click on the Cog symbol to view your Blocked Players List. This list is in reverse datal order, i.e. the accounts you blocked the longest time ago are at the top. Delete some of the older blocked accounts (as they’re likely to be banned by now if they were gold-selling spammers) to make room for new ones you want to block.
If you view the Blocked Player List in-game, it only shows around the top 10 (oldest) blocked accounts on-screen. There are, however, more blocked accounts because, if you clear those displayed, then refresh the list, it’ll show you the next 10 blocked accounts and so on.
If you’re playing on PC rather than on a phone, you have an alternative way of viewing the Blocked Player List, from within the Battlenet Launcher. At the top-left of Battlenet, click on the small blue swirl icon, and from the menu choose the first option of Settings (which has a little cog next to it). From the Settings window, choose Friends & Chat and then View Block List. This will display a list of all (up to) 50 blocked accounts at once.
