Overriding the Blog username breadcrumb in the native Blog module
A node preprocess function in your template.php file would probably do the trick:

function MYTHEME_preprocess_node(&amp;$vars) {
  if ($vars['node']-&gt;type == 'blog' &amp;&amp; $vars['view_mode'] == 'full') {
    $bc = function_to_get_breadcrumb();
    drupal_set_breadcrumb($bc);
  }
}


This should happen early on enough in the page build process for the changes to make their way through to page.tpl.php. You could also look at doing this in hook_node_view() in a custom module if you'd prefer.