Yeh, if it were charring, it would be very obvious in the chamber, especially since it would have had to burn down through the draft and into the stem. Not likely to happen.
Briar isn't necessarily absorbent, but it can have holes or pores. I've seen pipes that have been smoked so much that a pore in the birdseye will ooze tars when smoked from years of it building up inside the briar. It could be something like that, maybe a hairline fracture that is letting juices soak down into it.
or, it could be completely cosmetic and a problem with the stain.
I'm sure that someone else will ring the bell on this one.