Hey there!

I'm playing with blur effect on my Plasma 5.25.3 and seem not to understand how it works.

Use case/example: setting blur on/off in konsole settings doesn't affect anything, yet solely depends on global Plasma setting.
If 'blur' effect is enabled in settings, it is ALWAYS blurred, even if it is turned off in Konsole profile settings.
Similarly, if 'blur' is turned off in settings, it is NEVER blurred, even if it is turned on in Konsole profile settings.

Can I somehow get this separated? I want the blur on global scale, but I don't want the Konsole background to be blurred.

Can't duplicate that. The blur effect setting in konsole only applies to the profile it is set to. You can set a certain profile not to use blur and another with. The blur doesn't apply to all profiles. Only to the ones in which it is enabled. If you right click konsole>edit current profile>appearance>edit>blur on/off it will set it to the profile in use. If you should switch to another profile, the settings could be different. The blur setting in konsole is independent from the global blur setting.