I've not heard the expression you describe belli.

I've heard of topics being a "grey subject/or grey area" when the right/wrong aspect of it is not obvious.

For example, we're always told we should not lie, but, many people will not hesitate to say "what a pretty baby" to new parents because this is considered a grey area in the rule. Do you tell the truth and hurt the parents needlessly, or do you tell a lie and break the rule.