In addition, some foods (such as spinach) cooked on bare cast iron will turn black.
How true is that and why? Secondly, does it even apply to the newly seasoned cast iron cookware?
Spinach contains oxalic acid which reacts with cast iron and carbon steel pans turning the spinach black.
I'm not sure about your second question, I would suggest trying it and seeing what happens. My hunch would be that as long as you have a good seasoned coating on the pan it should be fine. But that's only supposition.