How to fix iPhone 6 Plus dark screen afte water damage

Do not put the iphone in rice after water damage. The best thing you can do is take it to someone who can repair phone as soon as possible or fix it yourself (And of course drain the liquid out). Rice will only make the problem worse.

This problem is most likely only the screen so just replace the screen and give the motherboard a good bath in 99.9% Isopropyl Alcohol then it should work fine.

If you know what you are doing, you can attempt to fix the screen by removing the backplate and very carefully rubbing it with Isopropyl. This sometimes fixes the screen but you will likely have to replace the screen anyways.