Since Doxygen supports the markdown syntax, writing a block quote in src comments (and md files) would be done like so

> This is a block quote
>
> -- Anonymous

Note the space between the > and the quote's text is required per Doxygen's manual on markdown support (see link below).

In doxygen's XML output, this would look like

<para>
    <blockquote>
        <para>This is a block quote.</para>
        <para><ndash/> Anonymous </para>
    </blockquote>
</para>

Currently, it seems like breathe ignores the blockquote tag entirely (during XML parsing of para tags). Just for completeness, the HTML output from an equivalent rST block quote looks like

<blockquote>
  <div>
    <p>This is a block quote.</p>
    <p class="attribution">—Anonymous</p>
  </div>
</blockquote>

Clearly, An attribution in a block quote also needs to be accounted for.

Some helpful links

• rst specs on block quotes
• doxygen manual on markdown syntax using block quotes