<route name="blog" pattern="^/blog" module="Public" action="Blog">
 <route name=".list" pattern="^(/)?$" action=".List" />
 <route name=".view" pattern="^/(post_id:\d+)">
  <route name=".viewpost" pattern="^(/)?$" action=".ViewPost" />
 </route>
</route>

form generated in template called ViewPostSuccess

<form method="post" action="/blog/2">
<fieldset>
   <ul class="comment_form_list">
   <li class="comment_form_list_item">Your comment: <textarea name="body"></textarea></li>
   <li class="comment_form_list_item"><input type="submit" value="Post your comment"/></li>
   </ul>
</fieldset>
</form>

Submit asks for ViewPostInput template which doesn't exist...