If you send the PDF to an offset press the black will overprint and the effect you are seeing with overprint turned on is accurate. If you want black to knockout make a new black swatch and add some amount of CMY to the mix or select the object and set it to knockout via the Attributes panel
Yes, that is normal. Acrobat displays K 100 as it will be printed, slightly transparent. If you want your black to knock out, just lower its value to 99,9%.
Thanks! Your answers helped me understand the issue. For my part I think just creating a custom black swatch seems like the simplest solution.
For anyone picking up this topic these two links may help: