>I have to create this field using IIF expression:
>LogicalFiled=IIF (somecondition,.t.,.f.)

You do not need an IIF() if you are returning a .T. or a .F.

LogicalValue = somecondition

>My question is: How can I make this field blank, if !somecondition?

Why do you want to? If its blank its .f., but if you really really want to, I think you can use the BLANK command