mrjeeves The difference is that OLED Dark is black. Normal dark is just dark.
The reason it's black is that it saves battery on mobile devices with OLED screens. On OLED screens, black pixels actually turn off rather than 'go black' meaning that no power goes to them, thus saving battery!
There is reasoning behind the dark!
Yeah a normal non OLED display has a backlight which provides all of the light for the whole display, and then the pixels are on top allowing a variable amount of the light through. So for those it doesn't matter if the background is grey or black as the backlight has to be on for the text to show, and the rest of the light it produces is blocked as much as possible. With this you don't really get a true black at all as some light always gets through, hence the background is normally set to a dark grey as it's consistent across the display, where with black the light leakage varies across the screen and can appear patchy. Power usage is pretty consistent as it's a single backlight level at any given time.
OLED each pixel is individual self lit, so there is no backlight at all. With this it means pixels displaying black are actually switched off, whereas ones showing grey would be drawing power. Hence power use can vary hugely based on how much of the screen area is light or dark at any given time. Since pixels turn off it also means they are true black which looks a lot nicer than dark grey on these screens too.
Hence both Dark mode and OLED True Black mode are required to cover both types of displays. At the moment it's mostly mid - high-end phones, high-end TV's (not relevant here) that use OLED, however there are some high-end laptops starting to move over too, so while a mobile version is probably the main thing of importance (especially with battery life) their is value in a desktop version too.