Blackmoor was the second supplement published for the original "white box" Dungeons & Dragons. It was first published in 1975. David Arneson was the sole author.

Despite the name, the supplement in of itself was not a tour of Arneson's fabled campaign setting. It was strictly a set of additions: new character classes, new monster, and new rules. Much of the material, no doubt, came out of Arneson's Blackmoor campaign. Hence the name.

Blackmoor added the Assassin and Monk character classes. It also added a "Sage" NPC class.

Blackmoor's biggest page waster was no doubt the inclusion of an entirely cumbersome hit location combat system. The idea was your character's total hit points were really an average. Your base hit points were used with a formula to determine how many hit points your head, right arm, left arm, chest, abdomen, right leg and left leg had. On a successful hit one had to compute where the hit landed. But that in itself was no simple calc. You had to factor in things like what kind of creature one was attacking. Humanoid? Fish? Reptile? Avian? And then you had to work in height factors and the length of the weapon used. (For example, no matter how hard you try, if you're a halfling with a dagger you're never going to land a head blow on a Storm Giant). But wait, there's more. If you reduced some body areas to 0 Hit Points, like a leg, the character or creature wasn't dead. It just produced game effects like a very, very slow movement rating. However, if you reduced the head to 0 hit points, you scored a kill. The system was far more realistic but the amount of record keeping made the whole system unusable. It's doubtful any gamer ever seriously used the system.

Arneson also introduced monsters and rules for aquatic adventuring. Arneson himself was a huge a fan of naval warfare. He authored of his own set of naval war rules, Don't Give Up the Ship. From these rules we can actually trace back the Hit Point concept.

There were over two dozen new aquatic monsters in Blackmoor. There's a perfusion of "giant" versions of prosaic sea creatures that approaches the comical: giant frogs, giant squids, giant crabs, and giant otters (yeah, giant otters!). More interesting monsters also made their first appearance, like Mermen, Aquatic Elves, and the Sahuagin.

There was also the inclusion of Temple of the Frog, D&D's first ever published module (see Palace of the Vampire Queen for a discussion on "first"). This scenario was based around some weird cult that was breeding a race of super frogs to populate the earth. There were a number of rings the players had to acquire and figure out how to use to gain access to various parts of the temple. The module also has a strange sci fi twist to it involving a high priest who is really a space man armed with some sci fi artifacts. The sci fi element surely invites some comparisons/links to the Expedition to the Barrier Peaks module.

The last portion of Blackmoor was rounded out with a rather simple but well thought out set of rules how to introduce diseases into the game. Clomping around in swamps with open wounds and never becoming infected wasn't entirely realistic (and like a fire breathing dragon is…). Arneson covered about a dozen infectious diseases, from athlete's foot to small pox. Depending upon what terrain type and season the characters were clomping through, they had a certain percentage chance of coming down with some disease. The rules also listed what kind of magical and non-magical cures were available.

See also for additional supplements:

I - Greyhawk, III - Eldritch Wizardry, IV - Gods, Demi-Gods & Heroes, V - Swords & Spells