External display black on rMBP 11,1 I have a Dell LCD monitor connected to my Macbook Pro with Retina display (late 2013) via Mini DisplayPort/VGA adapter. It was working fine at the end of last week when I took my laptop home. When I came back to the office this morning, I have the following symptoms:


Powering on both devices from cold powered-off state, my Mac boots normally and the LCD power indicator goes from amber to green as normal as it detects a signal
The LCD backlight turns on
The display is completely black
OS X (Mavericks, 10.9.4) detects the display, because I can arrange the two screens in System Preferences and even move my mouse pointer off the internal display
I can even change the resolution of the external monitor (when I do, the power indicator goes from green to amber back to green as it re-syncs)
The LCD panel's menu works fine, so the panel itself is OK


I have tried:


resetting SMC and PRAM
changing resolution of the external
closing the lid of the MacBook Pro
booting into the Yosemite Beta on another partition
resetting external monitor settings to default


I think it's either the Mini DisplayPort adapter or something in the monitor's signal processing electronics? But until I can get either an additional monitor or another computer to check things...any suggestions?
You have a solid troubleshooting technique, so I'd agree the adapter, or the VGA cable are  likely culprits. Sometimes it's as easy as the cable not sitting well in the connector, and that could mean it connects the necessary pins for detection/sync but not for actual display. 

One other thing I could think of is if the monitor has a source selector, i e if it has multiple inputs for VGA/DVI etc, and the right one isn't chosen. That's probably not the case though as you say it responds to resolution change.