I’ve been experimenting with using HTML. Add the following to your CSS snippets file:
blockquote.red { border-color:#f00; background-color:#f001; }
blockquote.green { border-color:#0f0; background-color:#0f01; }
blockquote.blue { border-color:#00f; background-color:#00f1; }
blockquote.magenta { border-color:#f0f; background-color:#f0f1; }
blockquote.cyan { border-color:#0ff; background-color:#0ff1; }
blockquote.yellow { border-color:#ff0; background-color:#ff01; }
blockquote.pink { border-color:#f9f; background-color:#f9f1; }
Then in your text:
<blockquote class="green">
Well done is better than well said. — Benjamin Franklin
</blockquote>

Unfortunately, this leaves the editor text a bit cluttered. A partial workaround is to hide most of the HTML. At the moment I can hide everything except the equals sign (after “class”).
div:not(.CodeMirror-activeline) > .CodeMirror-line span.cm-tag{ display: none; }
div:not(.CodeMirror-activeline) > .CodeMirror-line span.cm-attribute{ display: none; }
div:not(.CodeMirror-activeline) > .CodeMirror-line span.cm-string{ display: none; }
This hides the HTML unless the line is selected. And leaving the equals sign does give an indication that something is there.
Edit: I notice now that you can’t use markdown within HTML. So you’d have to use even more HTML for bold, italics, etc.