Oh, they believe it. It's just that their mission is to deny it and keep the evidence from seeing the light of day.
We really should stop looking at Blix as an incompetent buffoon. As witty as the line "Ignorance is Blix" is, it's now painfully obvious that it's not apropos. Blix is corrupt and criminal, and he knows precisely what he's doing. It's not a case of Saddam leading the hapless Blix on a wild goose chase or of Blix not seeing through Saddam's little games. Both are cooperating in this charade. It takes two to tango.
OK, maybe "Willful Ingnorance is Blix"