Why is there black residue on the aluminium, refillable deodorant case?Updated 2 months ago

It's oxidation, it's normal, and it's worth wiping off. 

Aluminium reacts with air to form a fine oxide layer — that's what stops the metal corroding, and it's why the case lasts. On the threads and moving parts, where the surface is uncoated and gets regular friction, some of that layer rubs off as a dark smudge. 

Left alone it can transfer onto the deodorant itself, so give the threads and lid a wipe with a soft cloth or paper towel now and then, and wipe the top of your stick if you spot any. 

Worth knowing: the aluminium in antiperspirants is a different thing — aluminium salts, formulated into the product to plug your sweat ducts. The case is solid metal. 

And if you'd rather not think about it at all, the case is optional — your stick works exactly the same in its cardboard tube.