Hey OP, that's not volcanic dust! That's water damage. Depending on the company, they wash out their trucks, once a week. They do this to keep the dirt/dust/road grime from transferring to the customers packages. Sometimes the contractors that wash the trucks leave the back of the trucks sliding door open to dry, before the morning shift starts loading their packages by destination. If it rains the night before, the trucks usually get the occasional puddle because of dents made by the daily abuse of heavy packages over the years. The shipping companies are notorious for this especially UPS and USPS. I used to work for USPS and it happened all the time. I bet you your package has water damage only on one side
If so, water seeped in and the black stuff you see is the moisture from the black foam that holds your MBP down in its plastic bowl/case. I would return it, so Apple can get their money back from the shipping company and help from others getting the same customer service.