"preseasoned" cast iron pot colors my broth I recently bought a certain "light cast iron" that looks like a wok. Cooking on induction, it seemed to give my pork broth a black tinge. Broth was fine, but there was certainly something from the pan. 

The wok is supposedly preseasoned, so I don't know why this happens. Any workarounds? Should I rub the inside of the wok with cooking oil and bake it in the oven upside down for 30 minutes?
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.