Need to add span to every blockquote using function

I want to be able to add a quote without having to add quote marks in the post/editor and without a plugin, so I was trying this:

In one template I have this:

<blockquote>
   <?php the_content(); ?>
</blockquote>

In the output I want this:

<blockquote>
           <p><span class="bq-start">“</span>... the content ...<span class="bq-end">”</span></p>
</blockquote>

What I was trying is this

function style_blockquote ($content) {

	$content = "<span class=\"bq-start\">“</span>" . $content . "<span class=\"bq-end\">”</span>";
    $content = str_replace("", "", $content);
    return $content;

}
add_filter('the_content','style_blockquote',0,1);

Of course this adds quotes to all content. I just want to add new tags to items within blockquotes. Can someone suggest a solution to a newbie?

Thanks