I'm a little vague on the details, but hopefully what I remember is enough to jog someone's memory. I think the story takes place at poker night at the loft. Brown or Rafe had commented either earlier or that night something along the lines of how he can't understand how Jim can handle living with Blair. In the end, Jim recognizes that he and Blair are more or less married. They may even be cuddled up on the couch. It's Jim's POV. And it had a light tone.
Thank you kindly.
ETA: Found, thanks to
ainm. It's
In Your Dreams by Livia.