Everything you need is in the email message headers array.
$params['headers'] = array(
'Bcc' => 'bcc_email@example.com',
'Cc' => 'cc_email@example.com',
);
Here is an example implementation of drupal_mail() with bcc headers included.
$params = array(
'body' => $body,
'subject' => $subject,
'headers' => array(
'Bcc' => $header_bcc,
'Cc' => $header_cc
)
);
$email = drupal_mail('ModuleName', 'message_key', $to, LANGUAGE_NONE, $params, $from, true);
Using hook_mail() you need to add (thank you @clive):
/**
* Implements hook_mail().
*/
function ModuleName_mail($key, &$message, $params) {
switch ($key) {
case 'message_key':
$message['headers'] += $params['headers'];
}
}