"preseasoned" cast iron pot colors my broth
Pre-seasoned doesn't generally really mean ready-to-use. It usually only means that the cookware is coated in wax and/or oil to protect it from rusting prior to use. It still needs to be seasoned. Consumer Reports gets into that and how to season cast-iron for first use. Seasoning is also covered at length on this site.

Don't worry though, the factory coating is perfectly safe to eat as you have.