@AndersHP
After main (240V) power on, it sits and waits for the button at the back to be pressed (because it's a latching switch and will still be in the OFF position).
Since this thread I have changed the monitor in my cab for a 12.1" one, which needs it's own power supply, so I now have 2 plugs - these are connected to an power extension lead.
So now when I power on at the wall (240V), the screen comes on for a few seconds, realises there's no video signal, and goes into standby mode. The Pi does nothing until I press the latching switch, at that point both the Pi and the screen spring into life.
The only time the pi will turn on instantly is if you have previously killed the power at the wall instead of using the switch. Then the switch will still be latched to the ON position, so when wall power is restored, everything will start up.
However I'd not recommend cutting power at the wall like that, as it's likely to result in corruption to your SD card eventually.
You might intend to use the software 'Shutdown system' option in RetroPie before cutting wall power, but that won't work, because as soon as the pi shuts dow, because the latched button on the Powerblock is still set to the ON position, it boots right back up again!
There's probably a way to do what you are trying - the Powerblock has points for connecting an additonal 5V device, so in theory if you had a 5v fade-in/fade-out dimmer, you could run that off the Powerblock and have the effect you want running from the Powerblock switch rather than the wall switch.
That's getting a bit beyond my abilities now though.