If the cells are truly blank cells then the following will work (pay attention to the first comment in the code):
Sub blah()
'first select a single cell in the column containing blanks you want to replace.
For Each cll In Intersect(ActiveCell.EntireColumn, ActiveSheet.UsedRange).SpecialCells(xlCellTypeBlanks).Cells ' this line will error i there are no blank cells at all.
If InStr(1, cll.Offset(, -1).Value, "AT-", vbTextCompare) > 0 Then cll.Value = "Austria"
Next
End Sub
If the cells are not truly balnk but in fact contain only spaces then this will work:
Sub blah2()
'first select a single cell in the column containing blanks you want to replace.
For Each cll In Intersect(ActiveCell.EntireColumn, ActiveSheet.UsedRange).Cells
If Application.Trim(cll.Value) = "" Then
If InStr(1, cll.Offset(, -1).Value, "AT-", vbTextCompare) > 0 Then cll.Value = "Austria"
End If
Next
End Sub