Blue will wear with normal use. I've had a Jap Browning that had a very worn finish on the blue receiver. VERY worn. Cold touch-up blue wouldn't fix it, either. It doesn't "take" on the Browning receiver steel.
The silver nitride will keep looking like new unless it's abused. It also won't rust when you carry it by the receiver on a hot day, sweat on it, and don't oil it immediately after. A blue one will. BTDT.
While I have bought used ones, I would not buy a new-in-box blue-receiver O/U.
Some "blue" guns like the light Guerinis are actually anodized aluminum. I'd get one of those. But blued steel is for hanging on the wall, not actual use.