Black greasy residue from stainless steel pot on lid only after simmering turkey stock for 18 hours. Question is the stock safe to eat ?
I'll assume that the inside of the pot and the lid are both stainless steel. 

The black gunk would be some sort of organic residue from the turkey or spices used. Often stocks are strained to remove the "solids" (froth) that result from the stock solution boiling. 

A turkey stock solution would typically have some fat on it. When bubbles burst at the surface some of the fat and froth would be "blown" onto the lid. This would form thye black greasy gunk observed. 

The stock, which had been simmering, would be perfectly safe to consume.