Actually it was in the iOS 7.1 beta, I believe. I think it was there in beta 1 and beta 2 as a setting under "Accessibilty" but then in beta 3 it got removed. Beta 3 however added the new fully blurred power off screen, round buttons, and round sliders. I personally loved those design decisions, aside from the removal of the awesome full screen call pics. But anyway, removing the dark keyboard really made me disappointed. Sure, I could go and get a third party keyboard, but I am perfectly satisfied with Apples keyboard. It's nice and steamline, looks pretty, I'm used to it, it has nice blurring effects (however they cause lag and stutter at times, ESPECIALLY on A7 and below iPads

) and its autocorrect is satisfactory. If I have an issue with something randomly getting corrected into something undesirable, I simply make a new shortcut in keyboard settings for the correct word.
I wouldn't be too surprised if Apple implemented a "dark mode" into iOS like they did in OS X Yosemite. They already have in a couple places, in iBooks for example, if you open the app in a dark space, or you cover the ambient light sensor, the UI goes dark and looks beautiful. There's also night mode in maps, as well, however I have heard that it doesn't work any longer as of iOS 8 for some reason? Maybe they have edited code pertaining to these things in preparation for a "dark mode", somehow breaking night mode in maps? That was a very roundabout guess but it's something, I suppose. Really hoping for a dark mode.