orchestral fade to black moments

I just watched the film for the first time in years and was wondering the same thing.

Is this something Krzysztof did often in his films, and why did he do this out of interest?

The way i felt it, it is the female character -and only her- closing her eyes and opening them again. The scene, when this happens while she is swimming, is one of my best of all times.
I've also never seen it before or since in a movie, but i find, it is the most ingenious way to put you -the viewer- in the mood and psychological state of the character.

My pet theory is that these scenes are a visual metaphor. Look at the dialogue right before a fade, they all evoke painful memories about her loss which she desperately suppresses. So the moment of black and loud music is the moment Julie needs to shut down her feelings of grief and when the scene fades back up she has warded off a breakdown and acts like nothing is wrong.

Here is one critic's take on it which I think is interesting:

Kieślowski settles on a fascinating aesthetic device to convey Julie’s memory: Whenever she is struck by a remembrance, we hear the striking chords of her husband’s composition and the screen fades to black for several seconds, effectively stopping time and shutting out everything else.

Fade outs in this film, which are traditionally used in movies to represent time passing or to conclude a certain scene, instead bring the viewers back to the point in time which the fade out began. The occasional fade outs and fade ins to Julie's character are used to represent an extremely subjective point of view.

According to Krzysztof Kieslowski: "At a certain moment, time really does pass for Julie while at the same time, it stands still. Not only does her husband's classical music come back to haunt her at a certain point, but time stands still for a moment."

I agree with what everybody else has said here, that those moments are moments when she is taken through her mind, through her memories, and into the music. She hides there, she contemplates or mediates there, she struggles with her past and future there...

...but I would add that each one is followed by a profound decision that leads her further down her path. She "composes" the next section of her life based on what she hears in the inner recesses of her mind and her memory. Each fade-out (if I recall correctly) heralds exactly such a moment.