I have asked online about rules of original Blackmoor campaign, and got directed to the game called "Dragons at Dawn". Review incoming.
Firstly, elephant in the room - how authentic is it? I don't know for sure! If you use "First Fantasy Campaign" and "Adventures in Fantasy" as sources it's highly authentic, but we also have character sheets from the original campaign, and they show characters having 30 or so skills instead of normal characteristics and then skills (like in DaD or AiF). Truth is, Arneson changed the rules a lot during the game.
Anyway, "Dragons at Dawn" is way better than ODND and makes me think about what could have been. Not all of what I am going to list is an advantage though, some if it is an archaism.
1. It uses almost exclusively d6, save for some class skills requiring d100, which let's be real is very close to ODND.
2. From the beginning it used ability-based saves instead of old-school dnd way, and also had a skill system.
3. Classes were very loose, almost non-existent, and multiclassing was the norm. This is highly authentic because Arneson himself said this.
4. Most classes were non-combatants, especially "exotic" classes like Merchant and Sage.
5. Combat system was simultaneously simple yet complicated because it involved only one die roll but also a lot of math, I am still not sure how does it work.
6. Interesting magic systems. One is elfin, which is based on songs, but basically it's just usual magic with mana. Another one is human, based on creating magic items.
In conclusion, OD&D could have ruled much more, but today I won't play "Dragons at Dawn". Today we have NSR and PbtA games - much better