A Reddit 'Shower thoughts' post:
If you borrow someone's newly bought blue cheese, you would have something old, something new, something borrowed, and something blue all in one item.
You could go to a wedding with that one item!
I'm guessing there's some pun or other non-obvious meaning behind 'blue' here, likely some contrast to 'borrowed' but I don't think I've ever encountered it in such context. What's so special about 'blue' that it got included here that the post raked up 17,500 upvotes?