I see this as yet another case where, if there are two possible reasonable interpretations of the rules, then you go with the FAQ interpretation because that's the one that WOTC put their official seal of approval on. Only if the FAQ intepretation cannot be right because it is not one of the reasonable choices for how to interpret the rules should the FAQ answer be discarded.
But that is not the case with this example. An illusion might gain the benefit of concealment from being blurry, and it might be that the blurry part is also part of the illusion and therefore it should gain no benefit. Both positions are reasonable, and both can make sense depending on which analogy you depend on, which reasoning, etc.. So given there are two reasonable intepretations, and the FAQ chooses one of those, I am going with the FAQ one.