For a consistency that feels like thick, sticky mud or bitumen, I recommend the following quantities:
500 liters of molasses (base)
25 kg of cornstarch (less than in the solid recipe to keep it flowable)
8 kg of activated charcoal powder (for color and a slightly "earthy" texture)
Optional: 20 kg of healing clay or fine bentonite clay (this gives it the perfect muddy heaviness and matte finish). Have fun but this substanz is very very messy. I think the substanz is how in this video here https://qs4ever-lab.umd.net/download_info/lucy-kitty-in-black-tar