BuddyPress.org

This one is probably better:

function profile_target_blank( $field_value, $field_type, $field_id ){

    if($field_type == 'url')  { 
	
          $field_value = str_replace('rel="nofollow"', 'rel="nofollow" target="_blank"', $field_value);
   
    }
	
    return $field_value;
}
add_filter('bp_get_the_profile_field_value', 'profile_target_blank', 11, 3);