Hey there LoadoutLord!
I totally get what you mean about seeing it in games like Metro, looks kinda wierd, right? But nah, it's not just some fancy marketing, it's pretty different from regular bread, especially your typical white or even whole wheat stuff over here on the East Coast.
The main differences? Oh man, where to start. For starters, it's usually made with a high percentage of rye flour, which gives it that dark color and a much denser, chewier texture than most of our breads. It's often sourdough-based too, so you get that really distinctive tangy flavor. And then there's the ingredients – alot of times you'll find caraway seeds in there, maybe even some molasses or cocoa powder to deepen the color and add a bit of sweetness. It's got a really rich, almost malty taste, very different from a light, airy sandwich bread. So yeah, definitely more than just a dark rye loaf, though rye is teh main ingredient. It's wicked good though, you should try it if you get a chance!