It doesn't do anything, it just... um... they know it's true?

[Ugh, this is awful.]

But then they can use the Blue Truth, and say something true in blue to refute your true claim.

[He holds his hands out awkwardly.]

Like if you say 'There are no doors to escape' they can say 'But there's a window' and then you'd have to say something like 'The windows are locked' in order to maintain that there's no escape.