I realized that the Blogger API allows labels, so I've coded up an
gblogger-add-label function, since it seemed like a useful thing. Here
it is:
(defun gblogger-add-label () "Adds labels to gblogger entry."
  (interactive)
  (let ((label (read-from-minibuffer "Tag? ")))
    (save-excursion
      (goto-char (point-min))
      (search-forward "</title>")
      (insert
       "\n"
       "<category scheme=\"http://www.blogger.com/atom/ns#\" term=\""
       label
       "\"/>"))))
I also added "atom:category" to the list of tags in blogger-edit-
post.xsl:
<xsl:template match="atom:category|atom:id|atom:issued|atom:modified|
atom:created|atom:published|atom:updated"/>
And it works! (for me anyway)
-- Joe