Breadcrumbs for category pages
If your categories are only one level deep, you could hard-code the breadcrumbs into the category template:

{% block breadcrumb %}
  &lt;ul class="breadcrumbs"&gt;
    &lt;li&gt;&lt;a href="{{ siteUrl }}"&gt;Blog&lt;/a&gt;&lt;/li&gt;
    &lt;li&gt;{{ category.title }}&lt;/li&gt;
  &lt;/ul&gt;
{% endblock %}


In _layout.html you would have:

{% block breadcrumb %}
  &lt;ul class="breadcrumbs"&gt;
    &lt;li&gt;&lt;a href="{{ siteUrl }}"&gt;Blog&lt;/a&gt;&lt;/li&gt;
    {% if craft.request.segments %}
      {% for crumb in entry.getAncestors() %}
        &lt;li&gt;{{ crumb.getLink() }}&lt;/li&gt;
      {% endfor %}
      &lt;li&gt;{{ entry.title() }}&lt;/a&gt;&lt;/li&gt;
    {% endif %}
  &lt;/ul&gt;
{% endblock %}


Haven't tried this myself, but I think it would work - the category template would override the layout. 

Of course, if you have multiple levels of categories, the intermediate levels won't be included, so it's not an ideal solution.