[observationist]

Bas relief and the play of light and shadow have been used since humans have tried their hand at carving. Seeing the impression a wet face left on dry cloth would be sufficient to tickle artistic inspiration, but actual artists, who spend their time thinking about how things appear and how to capture them in their respective media would have all sorts of opportunities for capturing the negative of an image, even if they wouldn't have thought of it in those terms.

There are plenty of examples of engravings, carvings, intaglia, and so on that used what we consider to be a "negative." There's nothing particularly special about flipping an image, transposing light and dark, inverting the 3d characteristics, or otherwise reversing different aspects.

Specifically, the inverse image might be carved for a wax seal ring or imprint, or it might be carved for a decoration stamp used in cement, or a mold for jewelry or ceramics. There are plenty of examples of things all throughout history that provide opportunity to inspire an inverted or "negative" image; it's simply our context of photography that is novel.