Destiny 2 is going to be the first non-Blizzard game to use Battle.net for online services on PC, and you can now link your Bungie account to your Blizzard one — which is timely, given that the Destiny 2 beta is set to hit consoles next week and PCs in August.
As a note, if you already have a Bungie account, you’ll want to be careful in linking things, as noted by some users over at the Destiny subreddit. That’s because Bungie accounts don’t actually exist as standalone things — they’re attached to an existing Xbox Live, PSN, or now, Battle.net account. So, if you want to link your Battle.net account to an existing Bungie account, when signing in, log in first through your regular PlayStation or Xbox Live account. Do not click the “Blizzard” option in that menu.
Then, once logged in, go to Settings -> Account & Linking, where you can add your Battle.net account to your existing account, after which point you’ll be able to log in with that as well going forward.
If you log in through the Blizzard option (shown above) before you’ve properly linked your Battle.net account to your existing Bungie account, it will create a new Bungie account tied to your Battle.net account instead of pairing it with your current Bungie account. The only danger then lies in if you then accidentally link your Xbox Live or PlayStation authentication to that new blank account. Doing so will orphan your original Bungie account with no method to log in.