According to the documentation :
To test specifically for a blank value use
if( Value = Blank(), ... instead of
IsBlank function considers empty tables as not blank, and
IsEmpty should be used to test a table.
But, this is not the issue. As
BookingCalendarNetwork is your SharePoint list,
varBKID is not part of the list. So you can not add a delegated filter like
varBKID = Blank() or
IsBlank(varBKID) to the SharePoint list.
You can try to rewrite your filter like that:
If(varBKID = Blank(),
Filter(BookingCalendarNetwork, ActiveStatus.Value = "Active" And 'Network ID' = ThisRecord.ID),
Filter(BookingCalendarNetwork, ActiveStatus.Value = "Active" And 'Network ID' = ThisRecord.ID And 'Booking Calendar ID' <> varBKID))