I have AMD Radeon HD 6790 Barts. Will it cause black screen? Should I just install official driver from their website? Or should I not touch graphics driver at all, because Dolphin emulator works. But I want to try CEMU and RPSC3 just to test it.
You are asking for opinions. One will say yes, the other will say no. In the end YOU will need to decide yourself.
Advice: prepare for failure . Investigate how you can revert changes you made that made the system fail.
Anything can be reverted from a live session or from grub rescue prompt. Just make sure you know what to do (as you likely will not have an internet connection from that system when it crashes ;-) ). When you are confident whatever option you pick you will be able to restore to a working situation.
Also another piece of adbvice: make backups. If you consider your personal data important you need to treat it as such, and the way to do that is to make backups at a regular interval. Incremental backups take little time to make.
Now back to your questions:
Should I just install official driver from their website.
If it aint broken don't try to fix it. But if you need it for a specific software that is a valid reason to try it.
But I want to try cemu and rpsc3 just to test it.
AMD Radeon HD6790 is an older card and had excellent support up to at least 16.04. So I would assume it is going to work. So if you want to test those and need to 3rd party driver: go for it. It is likely to work. And if not it is always possible to revert back to the old driver. Find out how to do that before attempting to install the new one and you can play with it until you are satisfied.
As an alternative: you could create a dual boot with 2 Ubuntu systems and tinker in the 2nd one keeping you current one as is.