This article might explain the problem you're having:
http://manual.b2evolution.net/Log_in#Issues_With_Logging_In

i check it and it had the correct base url alrady in there
$baseurl = 'http://intranet/blogs';

This is a cookie problem and you seem to be using version 0.9, are you?

Search for the line that sets $cookie_domain in /conf/_advanced.php and replace it with the following:

$cookie_domain = ( strpos($basehost, '.') ) ? '.'. $basehost : '';

The problem is that with hosts that have no dot in it (like "intranet") cookies won't get accepted, if prefixed with a dot. In 0.9.x the check there is (currently) just for "localhost".