Adapt something like:
Private Sub Worksheet_Change(ByVal Target As Range)
Set a1 = Range("A1")
Set t = Target
If Intersect(a1, t) Is Nothing Then
If a1.Value = "" Then
wasblank = True
Else
wasblank = False
End If
Exit Sub
Else
If wasblank And a1.Value <> "" Then
MsgBox ("changed from blank to nonblank")
wasblank = False
End If
If a1.Value = "" Then
wasblank = True
End If
End If
End Sub
This remembers what WAS in A1.