As a child growing up in a Christian environment, I was always fascinated by the concept of a "city of refuge," though I recently realized the only way i've really heard anyone discuss it is as a metaphor. It's certainly a visually striking concept - an innocent man, running for his life from a righteous but misinformed executioner, with a place of legal safety as a destination. Of course, just because it's an interesting or engaging idea doesn't mean it ever actually existed.
Did cities of refuge exist in ancient Israel? In similar cultures? If so, who were the "avengers of blood?" Is it simply a term for any family member or friend who'd want to take revenge, for a legal authority, or something else entirely?
I can't speak to Biblical cities of refuge (or similar cultures,) but you might find Hawaiian Temples/Places of Refuge interesting. The most well known is Puuhonua O Honaunau, on the Big Island. It was restored and turned into a National Park. The basic idea was that anyone that entered the place of refuge couldn't be pursued.
In Hawaii, there were an awful lot of things whose punishment could be death. In Hawaiian, forbidden things were called "kapu" (think "taboo".) I'll back up for a second. Hawaiian society wasn't properly unified for quite some time, so making blanket statements is dangerous. However, there's commonalities between different islands at different times. All that said, everything they say about victors writing history applies even more to societies that recorded their history through memorization and oral recitation.
There was generally two classes of powerful people, the Ali'i and the Kahuna. It would be really simple to think of the Ali'i as being the aristocracy and the Kahuna being the priesthood. Naturally, it was far more complicated than that, but hopefully that gives you just enough context.
The Kapu system laid out a variety of behaviors and actions that were forbidden. Either because they were thought to bring punishment from the Gods, or because it was considered an insult to the powerful (which also angered the Gods.) Everything from eating with the opposite sex, to breaking the shadow of the King, to fishing/hunting in the wrong area could be punishable by death. Not to mention being on the losing side of a battle or innumerable other things. Anyone who made it to a Place of Refuge was considered untouchable by both men and Gods. A Place of Refuge was run by Kahuna, and anyone inside was off limits, even for the locally powerful Ali'i/King.
I apologize that this doesn't directly address your question, but your question reminded me of the following passage in The Legends and Myths of Hawaii by King Kalakaua in 1888:
These places of refuge, with the right of circumcision, which existed until after the death of the first Kamehameha, suggest a Polynesian contact with the decendants of Abraham far back in the past, if not a kinship with one of the scattered tribes of Israel.
I don't know of a single modern historian, geneticist, or other academic that takes that idea seriously. However, the "refuge" concept is similar enough that someone trying to make Hawaiian history more palatable to a Western audience in the 19th century would give it a shot.
hi! it may be worth x-posting this one to /r/AcademicBiblical