When I tried, a blank date is imported as "00/00/0000", which is internally stored as "00000000". So the filter condition DATE=="00000000" works for blank dates.
You could let IDEA define the criteria by right-clicking on a blank date and then select "Display all records containing...." and see how the criteria is constructed.
@IsBlank may not work.
When I tried, a blank date is
When I tried, a blank date is imported as "00/00/0000", which is internally stored as "00000000". So the filter condition DATE=="00000000" works for blank dates.
You could let IDEA define the criteria by right-clicking on a blank date and then select "Display all records containing...." and see how the criteria is constructed.
@IsBlank may not work.