While testing http://moodle.org/mod/forum/discuss.php?d=114466
I've detected that something in blogs is causing wrong output of contents. After some reseach, it seems that the blog_print_entry() function performs a format_text() with contents twice:
$template['body'] = format_text($blogEntry->summary, $blogEntry->format);
with the second call, breaking the output generated by the first, noticeable <object> tags and so on from filters.
So I'm going to erase the second call, outputting directly $template['body'] that has been already processed.
For QA: After being applied, filters generated content (multimedia, multimovie...), should be present in output. Now it isn't.