The following CSS can be used to style the CKEditor blockquotes HTML. 

blockquote {
  padding: 0.1em 0.5em 0.3em 0.7em;
  border-left: 11px solid #ccc;
  border-right: 5px solid #ccc;
  margin: 1.7em 0 1.7em 0.3em;
  overflow: auto;
  width: 93%;
  background-color: #F5F5F5;
  }