Can Firebrand discard cards with out shield marks on them to use as a shield card. Only reason I ask is because cards can be used as shield cards, not they are shield cards prior to use. This and his wording seems to make the timing questionable as to if a card without shield marks could count as a shield card.

No. The RRG "Shields" section specifies that a card can only be called a shield card if it has one or more printed shield icons on it, so it must fulfil this criteria before Fireblade's interrupt can trigger. I've also confirmed previously with the design team that this is the intention.