The default when digitizing and saving the data from the Digitized Coordinates window to a BLN file is to Assign NoData (blank) inside the boundary (a BLN flag of 1). If you wish to Assign NoData (blank) outside the boundary when you save the BLN file, click Options | No Data Inside Region in the Digitized Coordinates dialog to uncheck that option. Then click File | Save and save the BLN file. The BLN flag value will be 0.
Updated October 18, 2021