Styles come and go, but a sleek black bag isn’t going anywhere. Just ask Nicole Kidman, who proved that the right black crossbody goes beyond trends, and makes any outfit appear expensive. Her exact bag is in the five-figure price range, but we found the look for just $23 — and it’s selling out. Kidman arrived at a pre-Met Gala dinner looking like she stepped out of a Parisian fashion magazine. Her all-black getup was classy, but the real scene-…