The Grid Blank command was renamed in Surfer 15; it is now called Assign NoData. This command is available from the Grids ribbon bar tab, in the Edit section:

For more information on using this command in Surfer 15, see:

• Surfer Converting Areas in a Grid File to NoData Training Video
• Can I fill outside a polygon to mask the map outside a boundary in Surfer?
• How can I grid only the area inside my data limits using Surfer?

Updated October 23, 2018