Anthony,
Well, if 'blocked stock' is excluded from ATP check, (OVZ9; it is common to exclude blocked stock), then no stocks or supplies marked with 'S' are used in the ATP check. This means that NO sales order (including your original sales order) will be able to use this Purchase Order for confirmations.
However, blocked stock is also by default excluded from MRP calculations (OPPI). If a purchase order were to be created with a blocked stock indicator, then MRP would most likely re-create an additional purchase requisition for the original sales requirement, which would now be considered to be a shortage, and un-satisfied by supply during the net requirements calculation.
You could answer such questions yourself by testing your concepts in a sandbox.
Best Regards,
DB49