Ah finally!!!! Solution found!

Here http://limulus.net/adhoc/2006/11/04/b2evolution-and-textmate-blogging-bundle/ I learned that the blog ID has to be added this way

 /blog/xmlsrv/xmlrpc.php#12

Then, important, to avoid 301 redirect errors, don't use

http://username@domain.com/blog/xmlsrv/xmlrpc.php#12

but add the www

http://username@www.domain.com/blog/xmlsrv/xmlrpc.php#12

Now it works! Yeah!