I am looking for an acrylic that is black on the outside but engraves white. Is that a thing anywhere?
Yes. Suppliers like Rowmark - Johnson Plastics Plus - as well as Inventables have this item.
Yes, this is a commonly available product.
Johnson Plastics makes a number of multicolor acrylic products, including the popular Rowmark brand. Here’s a link to a few of the possible options: Plastic | Laser | Sheet Materials | Engraving
Lots of other places sell similar products.