Suffered by the same "blackout" issue for both iPhone 6s and iPhone 7 plus, though it happened when I tried to power on the phone and there was no response for less than 1 minute. Fortunately the phone would work again after several trials. My best guess is some bug of iOS 11 while hoping it would be resolved sooner. Btw, the iPhone 7 plus frequently sticks on black screen with spinning wheel after upgrade to iOS 11 by OTA. Though these issues are tolerable so far comparing to the effort needed for a complete backup and recovery, which was indicated to be ineffective.