I am not sure what is the requirement to develop the interface for blanket release when you have the standard feature to load the blanket agreement's through spreadsheet through buyer work center page.
Please review the following note id What Are The Guidelines For Loading Agreement Lines - Spreadsheet Data - In The Buyer Work Center? (Doc ID 1265891.1)
Please check the following note. It has sample scripts for all scenario supported by Purchasing Document Open Interface PDOI:
How To create/update a Standard Purchase Order/Blanket Purchase Agreement/Quotation through PDOI process (Doc ID 1406003.1)
Replace the sample data with your actual data.
PBWC can be used to load Global BPA.
Use PDOI to load local BPA.