That appears to be some sort of nasty contamination bleed. Have them try a poultice please.

Joseph, what is a contamination bleed, in this case specifically, that would cause a black line, do you think?

There is another thread of someone else’s white quartzite getting more and more clusters of black spots within the stone though the owner says no adhesive was used in those areas.

Dirty hands of the guy making the seam? Could be. Hard to say. Fair callback though.