I often see comic panels where the background is blacked out or there is very little detail, but I am not sure what the conventions on this are and when we can do this or when it is typically done. It seems like it's used during inner monologues, but I think I've seen it in other situations, so I am wondering what are the rules on the background we can use, and when we typically do this.
Abstract backgrounds alone usually don't represent inner monologue. The usual convention to differentiate inner monologue from verbal speech are either thought bubbles (more prevalent in comedy) or captions (more prevalent in drama):
Depicting a character in front of an abstract background instead of one depicting their environment is often used to highlight the character and their emotional state.
Often it symbolizes that the character is so caught up in the moment that they no longer perceive their environment but only what matters to them in this particular moment.
Abstract backgrounds can be used during a fight scene to show that the characters are so caught up in the action that the only things that matter for them are themselves and their enemy:
It can be used for a character experiencing strong emotions or getting carried away in daydreaming. Even during a conversation with another character:
It can not just be used to represent the emotional state of the depicted character, but also how another character feels about that character:
As you can see, comics are a versatile medium allowing a lot of creative expression.
So what is the conversion on when you should be using an abstract background? Whenever the emotions of the characters are more important than their environment. (Or when you are approaching the deadline. Abstract backgrounds are often quicker to draw than representative ones. So when a comic book artist seems to overuse them, then it can be a sign that they were forced to cut some corners.)