Oven temp too high, bottom of roasting pan burned. Salvaged grease but it’s dark. Will It taste bad?

It looks like it might be (dark brown is probably still good, blackened is not). But give it a stir and a little taste—if it tastes bitter and acrid, that's a bad sign. You can still wing a gravy though even without drippings—here are a couple different recipes, depending on what's in your freezer and pantry (let us know if you need help with that!):
https://food52.com/recipes/7483-vegetarian-mushroom-thyme-gravy https://food52.com/recipes/74239-gravy-with-bacon-and-tamari