Hi,
You are not sure about your question . There is no concept of blank PO in sap . There is a concept of Blaket PO in SAP . Blaket PO means will have limits and provides a facility for blanket type .
I here with provide a link of sap help which will tell u about the process .and also some SCN links for the same .
what is Framework order and blanket PO
Blanket Purchase Order - process
http://help.sap.com/saphelp_470/helpdata/en/35/26c019afab52b9e10000009b38f974/content.htm
How ever ME58 is the transaction which will be used to create POs when a PR is alreday assigned with a vendor .
http://help.sap.com/saphelp_470/helpdata/en/35/26c019afab52b9e10000009b38f974/content.htm
http://help.sap.com/saphelp_470/helpdata/en/35/26c019afab52b9e10000009b38f974/content.htm
ME59 will be used when you want to create automatic POs when there is a assigned PR available . THere are certain pre requisites for creation of auto PO .
the below sap link will be useful to you in understanding the process .
http://help.sap.com/saphelp_erp60_sp/helpdata/en/75/ee11e655c811d189900000e8322d00/content.htm
Regards,
http://help.sap.com/saphelp_470/helpdata/en/35/26c019afab52b9e10000009b38f974/content.htm