2021 Washington Quarter - Odd Reverse Coloration

Perhaps there was some exposure in the box, along with the rolls that led to some environmental toning that only effected the part of the coin exposed?

Just a guess. Wait for the others to give their opinion.

Would be interesting--and next to impossible--to trace and document every detail of this coin's journey--from the striking chamber, being dumped into huge mint poly bags, short-term storage at the mint, transit across states to commercial coin handling/rolling/boxing facilities, being dumped into a high speed rolling machine then boxed, and delivery to the bank--and storage at the bank. Lots of points along the way where things can happen.

Like the nickels black beauty those quarters can has same effect.

There are three majors causes:
1. contact with the sulfuric contamination in presence of the carbon dioxide. This could be in the mint process if they use some sort of cleaning substances which contain sulfites.
2. In the process of drying if the machine go to the 600 to 750 deg Celsius the nickel become black.
3. the box you have was expose to sulfite environment.

This will answer to anyone about the "black beauty".