I am a rather new Westpac Altitude Black customer myself. I have a brief dig around after signing into online banking and could not find an online messaging option too.
If you have tried the number behind the Black card, I can only suggest calling at a generally less popular time (e.g. 7-9pm? or 6-7am?). If it is urgent, easiest way is to drop into a branch. Annoying I know – if expectations are high in terms of customer service for their top of the range card.
I am assuming you are an existing cardholder? I am not an Altitude Black cardholder so can’t advise of there is another service for this, but having checked with a Westpac bank account, Westpac doesn’t seem to have an online messaging function for sending messages that I can find myself.
Would appreciate any other Altitude Black cardholders answering this one.