I'm using krita in a musical project and I need a simple way to do a fade to black effect when I finish live painting.

I know I can use a black layer and play with opacity, but I would like to do it with some automation and pressing a shorcut if possible.

is there any way to achieve that?

You can use opacity keyframes to give an opacity transition over time, from 0 to 255, for a black layer above the animated image layer:
https://docs.krita.org/en/reference_man ... curve.html
i.e fade-in the black.

Or, with a black layer under the animation, you could use opacity keyframes on the animation (single layer) at the end to fade-out to black.
Sadly, there is no magic button that will do it for you.