Back at the barns, there’s muck and mire. Then there’s Walter Blum, right. Hard to miss this guy, even at a Kentucky Derby filled with sharp dressers. Heck, what other exercise rider trades a mud-splattered helmet for a Dolce & Gabbana hat? And Versace shirt. Plus Prada shades. No faded chaps for this galloping chap. “Just trying to bring a little fashion out here,” he says. “There are a couple …