I installed Fallout London but all I get is a black screen.so I want to do a fresh install of both F4 and FL. What do I need to delete for a fresh install?

If you have any mods installed, you should remove those in your mod manager. Obviously, uninstall Fallout: London and Fallout 4 as usual, then delete everything in your Fallout 4 install directory (if anything remains there). After that, you should remove two folders:
Users/"YourUsername"/AppData/Local/Fallout4
Documents/My Games/Fallout4