I went to Checkers Burgers today an had the nastiest french fry worst thing i ever tasted....?

Ok so i bought fries an burger today. I was eating a fry driving an not paying attention. What i put in my mouth wasnt a fry but some nasty black goop i immediately spit it out. This was the nastiest thing i ever had to taste. I called their national number to complain. I never do this but it was that bad. A rep took my call an said someone will contact me back. Has anyone ever had something like this happen to them whats the next step. Will they actually do anything. Unfortunately i trashed the receipt but saved the nastiness that i spit out....