Many users are facing the problem that when they try to take a screenshot from their iPhone 7/8, the screen captured is all black which seems to be a bug of iOS. The problem is occurring everytime user take screen shot from iphone and saves as black screen capture only. Apple do not provide any updates on the issue but need to fix this ASAP.
Below are few possible fix that you can try :
Fix 1: close all opened apps
The double click home button thing to swipe up and delete apps has everything all black as well.
Fix 2: Hard Reset your device
Try to Hard reset your device with holding power and home button simultaneously until apple logo appear (hold for about 10 sec).
Fix 3: Let your phone die till battery drain
If none of above steps work for you let your phone run of battery and shut down due to this. Now connect iphone to charger, when you turn on the iphone the problem should be fixed.