compass/frameworks/blueprint/stylesheets/blueprint/_debug.scss

@mixin showgrid($image: "grid.png") {
  background: image_url($image);
}

@mixin blueprint-debug($grid_image: unquote("grid.png")) {
  // Use this class on any column or container to see the grid.
  // TODO: prefix this with the project path.
  .showgrid {
    @include showgrid($grid_image);
  }
}