Finally! Darker than lowest brightness setting on iOS 8

• Go to Settings → General → Accessibility → Zoom.
• Enable Zoom.
• Tap 3 fingers on the screen to bring up zoom options.
• Choose filter: low light.
• Go back to Settings → General → Accessibility → Zoom and change the zoom region to full screen.
• Turn Zoom off.
• Go to Settings → General → Accessibility → Accessibility Shortcut and set it to Zoom.

Now, whenever you want darker than dark triple click the home button and voila!

Thank you stackexchange!

http://apple.stackexchange.com/a/152746/99607