@mixin default-border-radius($prop, $args) {
  -webkit-#{$prop}-radius: $args;
  -moz-#{$prop}-radius: $args;
  #{$prop}-radius: $args;
}

@mixin block_mixin($myColor) {
  color: $myColor;
  .prop {
    font-size: 14px;

    .child {
      @content;
    }
  }
}

body {
  @include default-border-radius(border, 4px);
  @include block_mixin(blue) {
    margin: 10px;
    padding: 10px;
  };
}