Once you've installed BlueBoard, you can enable the tweak via the settings menu.
Once it is installed you may be surprised to find that the blue tint only applies to the actual keyboard keys, and not the entire keyboard overlay as a whole. Instead, the blue keys are flanked by the standard light grey background present in iOS 7. The light keyboard can be changed by visit the settings menu of this tweak. You can changed the light background via the settings menu.
Like I mentioned above, BlueBoard only effects the actual keys, but enabling the dark keyboard mode will tint the keyboard background.
BlueBoard is available for free on Cydia’s BigBoss repo. Let us know what you think about this tweak in the comment section below.