If it’s for styling purpose, the fastest solution I could think of is by using Javascript. As far as I know, modifying the output of TinyMCE tag would require editing of WordPress core files, so it’s more practical to append a <span> with Javascript. Something like this would do:
<script type="text/javascript>
$('.post blockquote').wrapInner('<span />');
</script>
Where .post is the class for your single post container.