Wow now, I've found out how to count BlogComments of a BlogEntry.
This is done by a nested search implemented in the "format" section of the
RenderBlogCommentCount TopicFunction and inserted into
the
RenderBlogTeaser which in turn gets used by the
SubjectCategories and
BlogAuthors survey pages. This has been implemented in a separat
formatting
TopicFunction for maximum flexibility to use it
in different places in the
RenderBlogTeaser,
RenderBlogComment
and
RenderBlogEntry.