If the bottom is actually "blackened" you do not want to use the pan drippings they will taste burnt. If it is deeply browned it should be fine, but it really is a judgement call.
You could move to a fresh roasting pan and finish your cooking there and then deglaze that pan.
If you still have neck, wingtips or/and giblets you can roast those and make a brown stock with that for your base.
If the bottom of the pan is really scorched then you can't do anything with it. The gravy will taste like charcoal. Taste the broth you added and see if it's okay. If it tastes burned, then it's time for plan B.
Make a roux with butter and flour, and use chicken stock to make the gravy. Do you have any giblets left? The tail? Wingtips? Simmer those as well to get more Turkey flavor and add them to whatever stock you do have. If you have a fresh roasting pan, transfer your bird into it to try to capture any potential future drippings. If not, the chicken stock is the best you can do, unless you can clean your current pan and replace the bird into it for the remainder of the time.
If the pan is just really brown but not burnt, you should be ok.
Are there any stores open nearby? If you don't have chicken stock on hand, then go get some, or some gravy in a jar. I agree that it will taste like burnt. But try it, maybe it's just well-browned.
We are using a grill for the turkey, and mom got packets of dry gravy mix that I'll combine with a quick stock I made out of the neck, onion, carrot, and celery.
Otherwise, try a vegetable gravy, browning some onion then proceeding with a roux/water, if nothing else.