Something Blue is Darcy's side of the story. Everything that happens in Something Borrowed is seen from her point of view, and then the story continues. I didn't think I would like this book. Darcy was so unlikable to me. I'm glad I read it. It's fun and I would still consider it "chick lit," but there's an undeniable message. It's a story of maturing and moving on with your life. And (spoiler alert) by the end of the book I loved Darcy. She grows leaps and bounds in the two years after the initial story.
Warning: don't see the movie Something Borrowed until after you've read this book. It changed a part from the first book that somewhat ruined Darcy's love story for me. Yes, she finds true love too, and it's wonderful.