Short answers, pulled from the story.
Dave Arneson's Blackmoor campaign began in the first quarter of 1971. The supplement bearing its name contains almost no actual information about the world he had built. This paradox defined the product as the booklet served primarily as a rulebook rather than a setting guide.
The introduction of the hit location system in 1975 marked a radical departure from the abstract combat of the original Dungeons & Dragons rules. Before this supplement, a character or monster had a single pool of hit points, and a successful attack simply reduced that total. Arneson's new system assigned specific hit points to individual body parts, meaning a character's height and weapon reach determined the odds of striking a specific area.
The supplement introduced two character classes that would become staples of the fantasy genre: the monk and the assassin. The monk was designed as a monastic martial arts sub-class of the cleric, intended to function as a hybrid of the fighter and thief classes. The assassin, meanwhile, was a sub-class of the thief, specializing in stealth and lethal precision.
Blackmoor added the first comprehensive rules for underwater adventures, a concept that had been largely ignored in previous editions of the game. The supplement provided guidelines for swimming, equipment weight restrictions when fighting underwater, and the effects of underwater combat on weapons and spells. These rules were necessary because the original game assumed all action took place on land or in the air.
The Temple of the Frog stands as the first role-playing game adventure ever published, appearing within the Blackmoor supplement. This module was revised and expanded years later, eventually being published as DA2 Temple of the Frog. The adventure was set within the Blackmoor campaign world and featured a complex dungeon filled with traps, monsters, and puzzles.
The Blackmoor supplement was reproduced as a premium reprint on the 19th of November 2013, as part of a deluxe reprint of the original White Box. This edition included Volume 1: Men & Magic, Volume 2: Monsters & Treasure, and Volume 3: Underworld & Wilderness Adventures, along with four supplements. Each booklet came with new cover art but otherwise reproduced the original content faithfully, including the original interior art.