Thats just an interpretation of fluff, not somthing that is excplicitly stated in the rules. The rules just say that psychich disruption increases the Threshold of all psychic powers within the area, while the rules for the daemon weapon doesn't say anything about the daemon having to roll power rolls for the daemon weapons' special powers to work. Therefore it shouldnt have to sufer any form of test or dificulty in working properly. And also if you read the text under Weapon Attributes it mentions the following:
Let me put it this way... if there was a low-power psychic effect generated in the vicinity of your Blank - should it really change anything logic-wise whether it was created by an extremely simple power roll or just automatically?
By the way: It's not "just an interpretation of fluff" - pretty much every fluff source there is about blanks says outright they suppress or diminish any psychic effects nearby.