When I install Manjaro in discrete mode, I can not boot it in dynamic mode and the other way around, I get blackscreen with backlight after selecting Manjaro in grub. I aready tried to blacklist nvidia in grub but it did not work, and the dynamic mode still did not load.
Welcome to the forum! 
Please don’t use a run-on sentence as a topic title. Be clear and concise.
Please define what you mean by discrete GPU mode vs. dynamic mode. There’s no such installation or boot option.
Help us help you. 
Please see:
It’s a laptop GPU system architecture:
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Dynamic mode
The display is connected directly to the integrated GPU and the discrete GPU is routed through the display via the integrated GPU. This is the most common laptop GPU system architecture.
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Discrete GPU mode
The display is connected directly to the discrete GPU and the integrated GPU is turned off. This architecture is targeted to enhance display-GPU syncing performance. Not common, but can only be found in gaming laptop.
@str, that’s a common problem for GNU/Linux distro. AFAIK, there’s currently no out-of-the-box support for mux switching architecture for any GNU/Linux distro. You need some manual intervention on the display server. So my advice is : either you use dynamic mode or discrete mode only, but not both.
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