At American weddings, read a traditional poem: "Old things, new things, borrowed things, blue things." How to understand?

This is a custom. It is customary to get married with an old one, a new one, a borrowed one and a blue one. It is said that only in this way can we be happy for a hundred years. Yi Shu, a Hong Kong woman writer, also mentioned "something new and something old, something borrowed and something blue" in her works, which is also based on this allusion.