Thanks for the info here, everyone.
My PS4 is my first foray into PlayStation Land (previously a proud XB360 player), so I wasn't aware of just how bad Sony's PS Store actually was. The patch downloaded for me yesterday which was quite exciting, since - like a lot of people here - I assumed getting it a day earlier than promised was to enable us to have access to the Black Emporium first thing this morning. I couldn't understand, therefore, why it didn't become available today, and I've been checking the PS Store hourly to see if the DLC appeared there. When it didn't, I began to assume there was (yet) another glitch in the game.
I'm pretty annoyed that it looks like I won't now get the DLC until a day after it was supposed to be available, but thanks to you guys, at least I now know the reason and where the blame really lies. I guess the internet does serve some kind of purpose, after all!