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;
}