Why do soy sauce stir-fry dishes that have been dried for 180 days turn black?

Because melanoidins are produced.

Soy sauce is oxidized to produce melanin and starch from the tyrosine in the raw protein. Amylase hydrolyzes it into glucose, which reacts with amino acids to produce melanoidin, which makes the soy sauce produce a bright and shiny reddish-brown color.

Soy sauce is a traditional Chinese condiment.

A liquid condiment brewed from beans, wheat, and bran.

Reddish-brown in color, with a unique sauce aroma and delicious taste, it helps stimulate appetite.