Hi,
The blanket PO's are used when the quantities / delivery dates are uncertain. Only the validity date has the control in blanket PO and GR is not tracked. Also, you cant enter multiple delivery date or delivery schedule for a blanket PO.
If you are certain about the delivery date (twice a week as explained in question), you may go for a scheduling agreement and create the delivery schedule accordingly. In this case, you can track the on time delivery and other analysis.
Regards,
AKPT