Why do people say aviation regulations are written in blood? I have heard it said that all aviation regulations are "written in blood". I think this means that every rule in the book exists because someone died - and there was a lesson learned. Is this accurate? Is there anything else to the story of this phrase?
No, that's all it means. Aviation regulations tend to be put in place in response to accident reports, where a plane crashed or almost crashed for lack of the regulations. So, if you're confused why a stupid-seeming regulation exists, there's a good chance that people died because of the thing the regulation changed.

Obviously, not all regulations are like that; for instance, noise limits are based on quality of life for those near airports, not crashes. However, a good number of aviation rules came into effect as recommendations following a crash report.