Since the crossplay patch, I have been able to send battle.net friend requests through the “groups” and “recent players” social menu tabs to players that I had no previous friend relationship with on PSN. However, I have not been able to send battle.net friend requests to existing PSN friends, unless I happen to know their full battletag and type it in manually.
When I look at the “friends” menu on PS4, I can see friends from both the PSN friend system and the battle.net friend system. When I click on a PSN-only friend, the menu gives me the option to “whisper”, just like for a battle.net friend, with no “add battle.net friend” option. If I try to send a whisper message, it fails with an error “The whisper recipient is not on your Battle.net friends list”. (Note that I have not had a chance to see what happens if I’m in-game with a PSN friend or have them in “recent players”, so I don’t know if this issue is specific to the “friends” tab or not. I will update with more info when I get a chance to do this.)
So, I think what must be happening is that the social menu is failing to distinguish between a console-platform-only friend and a battle.net friend. I can still send messages through the console messaging system of course, but that requires minimizing the OW client, which is problematic because I might end up queued into a game without realizing it or something. It would be really nice to be able to DM them from within the game interface.
Also, and I don’t know if this is intended behavior, the “social” entry on the main menu screen only seems to show the number of battle.net friends that are online and playing Overwatch. So, right now, I have to manually check to see if I have friends online most of the time, because most of my friends are pre-crossplay PSN-only friends.