Before authenticating with the ACS, the switch port is blocked, so, It does not matter if you use a local user (registered in the PC) or a valid user in the domain (registered in the Domain Controller), the only way the Authentication pop up window for the ACS appears is to have access locally to the PC. You cannot have access to the PC through the domain user and password because you cannot reach the Domain Controller and because you have not authenticated with the ACS yet and the switch port is blocked.