Well, cheat like I learned from a caterer. Use Caramel powder to darken the gravy as dark as you want. It is not a food color. Caramel powder is used to get dark color in foods. The powder hardens solid in the pantry so I mix it with water to make a paste and it takes about 1/16 or less teaspoon to darken gravy. Does not add any flavor only color. Used to make the black loaves of bread and the darkest chocolate cookies and frosting. One jar of powder it will last you for years. There is a liquid too that may be easier to find. I've never used the liquid.
Here is a blog about it:
https://www.friedalovesbread.com/200...d-copycat.html