Dell External Display suddenly stops working - Puzzling
Your issue is in Macbook's powering.  

If you unplug the power adapter and then reinitiate connection to the monitor by either turning the monitor on/off or replugging the Display Port cable, you'll see signal restored.
After that you can plug the Macbook's power adapter back again and it won't turn black for hours or even days (unless you use cheap "Chinese" power adapter -- that can blackout every 30 minutes or so). The only issue remaining is that if you use custom calibrated color profile after such reconnection the color profile resets and you have to open Monitor settings and just open the Color tab to reapply settings -- you probably can make an Automator script for that routine and bind a keyboard shortcut.

I've noticed that monitor can go black when my fridge in the next room turns on, so it's definitely about power. Seems like Macbook passes some bad electricity signals from charger to the Display Port.

"source": my comment to this thread http://en.community.dell.com/support-forums/desktop/f/3515/p/19658656/20922700#20922700