I am not sure, if I understood your question.
Why would somebody Close a Blanket PO? You cannot validate a Blanket PO so no question of closing it. Do you mean Cancel the Blanket PO?
In this case, the blanket po was closed by the buyer by accident. In general, the blanket po's cover a certain time frame, and then there are sourcing events to get the best price. The old blanket would be closed and a new one setup under the new supplier.
Ok.. Even though Blanket PO is closed/Cancelled, the PO created will not be closed ( I don't think so). May have to test it but I am positive the PO will still remain open.