Many users face the matter that once they attempt to take a screenshot from their iPhone 7/8, the screen captured is all black which looks to be a bug of iOS. the matter is happening everytime user take screen shot from iphone and saves as black screen capture only. Apple don't give any updates on the problem but need to fix this ASAP.
Here is how to Fix iPhone Screenshots bug capture black screens
1: close all opened apps
The double click home button thing to swipe up and delete apps has everything all black as well.
2: hard Reset your device
Try to hard reset your device with holding power and home button at the same time till apple logo appear (hold for about 10 sec).
3: Let your phone die until battery drain
If none of above steps work for you let your phone run of battery and shut down due to this. currently connect iphone to charger, once you turn on the iphone the matter should be fixed.