I know this is an old post and that the subject is controversial, but a blank means you know it is blank. NULL actually isn't a value. It's a condition. It means that you don't know what the value is.

I also think it's a problem that Excel has no default method of telling you that a cell is blank or that it is empty, which shouldn't be confused with NULL because empty cells are known to be empty whereas a NULL would mean that you don't know if the cell has a value (even if the value is "blank" as in spaces) or is truly empty. 😛