The default when digitizing and saving the data from the Digitized Coordinates window to a BLN file is to blank inside the boundary (a BLN flag of 1). If you wish to 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.
In Surfer 12 and previous versions, the blanking flag is always set to 1 when saving a BLN file from digitizing. You change the flag value manually after the BLN file is saved by clicking File | Open and opening the BLN file in the worksheet, set the blanking flag value of cell B1 to 0, and save the changes.
Updated October 18, 2018