Я не перед Visual Studio - но попробуйте что-то вроде этого.
routes.MapRoute(
"Default1",
"",
new { controller = "PostController", action = "Index", dynamicBlogName = ""}
);
routes.MapRoute(
"Default2",
"{dynamicBlogName}",
new { controller = "PostController", action = "Index", dynamicBlogName = "" }
);
routes.MapRoute(
"Default3",
"post/{dynamicPostName}",
new { controller = "PostController", action = "Default", dynamicBlogName = "", dynamicPostName="" }
);
routes.MapRoute(
"Default4",
"{dynamicBlogName}/post/{dynamicPostName}",
new { controller = "PostController", action = "Default", dynamicBlogName = "", dynamicPostName="" }
);