You raise a valid question.
The matter of bellows flair comes up from time to time. I spoke with Western Bellows about this some months ago and they indicated to me that it was not a matter of a verifiable nature.
I have three sets of bellows that they have made. Two for 8X10 Deardorffs and one for my 12X20 Korona. All bellows that they made were not of an absolute black nature on the interior. They seem to work very well.
My bellows on my Zone VI 4X5 are more black on the interior. I can not tell any difference with densitometric tests of negatives, using the same film and developer from any of these cameras.
I would go out and make photographs if I were you.