The Blackhawk .357/.38 Special/9mm Convertible is a superb choice; while a 9mm cylinder can be fitted to a .357/.357 magnum by Ruger, call first; as mentioned earlier, there can be some caveats.
My 1979-vintage stainless Blackhawk shoots both 158 gr .357 magnum and 9mm 124 gr like a house on fire-and with identical PPA/POI. It's a gun that in some ways doesn't make sense or have a discernible niche (except maybe for hunting, which I've used it for)-until you shoot it. It'a a magnificent revolver, and a pleasure to use. I've actually used it in both .357 magnum and 9mm in IDPA matches. I'm pretty competitive for the first six rounds-until a reload is necessary, then the wheels kinda fall off...
Mine is extremely accurate, durable and reliable. The beefy .357 and 9mm cylinders (enlarged to proportionally fit in the .44 magnum receiver frame window, not due to any ballistic necessity-for smaller/proportionately sized and lighter, go to a Flattop model) are heavy enough to constitute a deadly weapon intrinsically if thrown at someone...
Best, Jon