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.