My hunch is that is moisture getting into the stone. Mt. Blanc is fairly porous and the edges seem to be particularly prone to wicking in moisture. It happens with the Macaubus quartzite too. Others have posted about this, and you can read about it if you search Houzz for Macaubus or quartzite staining.

Hi Kelly, my sea pearl quarzite is staining as well with water marks creeping along the edges. Did you ever get yours fixed?

Hi!!! did you just get it installed? most of the water marks resolved after a week but at the joint at the sink, they remained a small dark spot and I eventually decided to get that piece replaced. The Quartzite has been great otherwise!

Professionally installed. They came back a week later and resealed and noticed the dark water stains around the rim the sink. A year later the staining has travelled/creeped extensively. They say there’s nothing they can do.

UGH! so very SORRY