MTS should be able to dig their way through any substrate that isn't solid. Black sands are usually ground up pummice and basaltic rock, and under the microscope, can be extremely sharp, and as hard as granit

. Yet MTS dig in it without a problem.
On a tangent, I once I did an experiment with MTS and an Ant Farm. I put some algae and flake food in the bottom, then filled the Ant farm with about three inches of pool filter sand, then another four inches of a dark aquarium sand. I then added water to the top of the sand, no higher, and compacted it as much as I could. I stopped after cracking one glass side. After sealing the leak, I put in ~10 snails small enough to easily fit, but large enough that I could easily see them.
From what I saw, it took them about 10 minutes to dig down to the bottom and get to the food. After about a day they had churned up the dividing line, in most places, such that the clear deliniation, of sand types, was gone. Instead there was an inch or more of gradation of color.
I didn't let it go on much longer, mostly because my curiosity had been sated, and I figured my crayfish would like to eat the snails, rather than just letting them slowly die off in the "Snail Farm".