When I write Markdown with Harp, this is how I style the blockquote. You can use it as a LESS or Stylus partial if you’d like.

_hb-snippet-md-blockquote.less

/* Blockquotes

   Corresponding Markdown is:

   > This is the blockquote
   > **Author [Source](http://example.com/article.html)**

   Replace the variables with your own.

*/

@font-family-base: Verdana;
@font-family-alternate: Georgia;
@color-text: #333;

blockquote {
  font-family: @font-family-alternate;
  font-style: italic;
  color: lighten(@color-text, 25%);
  font-weight: 400;
  margin-left: 0;
  margin-right: 0;
  &:before {
    color: lighten(@color-text, 40%);
    content: open-quote;
    display: block;
    font-size: 8em;
    line-height: 1;
    position: relative;
    text-align: right;
    float: left;
    margin-top: -0.1em;
    margin-left: -0.5em;
  }
  p:last-child {
    strong:only-child {
      font-style: normal;
      font-weight: 600;
      font-family: @font-family-base;
      display: block;
      &:before {
        content: "\2014";
      }
      a {
        display: block;
        margin-left: 1rem; // One rem should equal the width of the em dash
        font-weight: 400;
      }
    }
  }
}

_hb-snippet-md-blockquote.styl

/* Blockquotes

   Corresponding Markdown is:

   > This is the blockquote
   > **Author [Source](http://example.com/article.html)**

   Replace the variables with your own.

*/

$font-family-base = Verdana;
$font-family-alternate =  Georgia;
$color-text = #333;

blockquote
  font-family: $font-family-alternate;
  font-style: italic;
  color: lighten($color-text, 25%);
  font-weight: 400;
  margin-left: 0;
  margin-right: 0;
  &:before
    color: lighten($color-text, 40%);
    content: open-quote;
    display: block;
    font-size: 8em;
    line-height: 1;
    position: relative;
    text-align: right;
    float: left;
    margin-top: -0.1em;
    margin-left: -0.5em;
  p:last-child
    strong:only-child
      font-style: normal;
      font-weight: 600;
      font-family: $font-family-base;
      display: block;
      &:before
        content: "\2014";
      a
        display: block;
        margin-left: 1rem; // One rem should equal the width of the em dash
        font-weight: 400;