The blanket PO is a purchase order left open over a period of time so that multiple receipts in partial quantities can be booked. Of course, none of the receipted quantities have the correct timestamp. This causes confusion and error in many areas but especially exception monitoring in MRP gets out of whack. This is one of many things that causes the material planner to throw in the towel and curse this stupid SAP thing. Excel becomes the tool of choice!
A purchase order in SAP can have many line items to procure materials to a certain date in a certain quantity. Once the vendor delivers, you should post a goods receipt and close the PO. If the receipted quantity is less than what was ordered, you set the final delivery indicator which closes the purchase order and AP will pay the bill (for what you receved).
If you leave the PO open, as in a blanket PO, MRP doesn't know that there are missing quantites and will further plan under the assumption that enough material is available yesterday... and then yesterweek... and so on. You will receive the exception message 07 'finish date in the past' and another one to reschedule and expedite. Your availability checks will produce false results and if you perform vendor evaluation you will see 'funny' lead times which are very different from what you negotiated.