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?
Best Answer
The Reddit poster refers to a British custom, in which a bride is sometimes presented with "Something old, something new, something borrowed, and something blue." These are carried by the bride as good luck charms.
The custom is said to have its origin in a nursery rhyme:
Tradition holds that the "Something blue" represents fidelity, purity, and love. "Something old" represents tradition. "Something new" augurs a prosperous future. "Something borrowed" is taken to represent happiness. The "Sixpence in your shoe" is meant to ensure prosperity.
I imagine that the post garnered an extravagent vote total simply because it is a clever play on the wording of the custom and the rhyme.
A more scholarly digression on the custom and the rhyme can be found in this Wikipedia article.