Whether it will erase your data or not after fixing your iPhone black screen of death usually depends on the method you try. For sure! The most common method, Hard Reset, will not cause any data loss. And also you can try Passvers iOS System Recovery. It provides three modes to fix your iPhone software problems, including Free Quick Fix, Standard Mode, and Advanced Mode. The first two options will do no harm to your data, even the first one is totally free.
But, if you try to restore your iPhone with iTunes/Finder, your data will be cleared.