Friend List

Click the Social Menu icon, "Relationships", and click Add Friend button. Add the player's name and he's now part of your Friend List.
You can also add a player to your Friend List by directly double left clicking them and choosing Add to Friends option.
Once your friend is in your list, click him to see the following options: Private message, Block, Guild Invite, Send Mail, Delete.
To Blacklist someone, Click Block Player button. Add the player's name and he's now part of your Block list.
The only option available in the Block menu is the Delete a player from the list.