from the title page:
A Setting Supplement Compatible with the Delving Deeper Rules and all Editions based on The original 1974 roleplaying game
from the introduction:
Blackmarsh was designed to evoke the earliest campaigns of the world's most popular roleplaying game. It takes place amid a wilderness full of danger and the possibility of great treasure. It is organized as a numbered hex grid with locales keyed to specific hexes. Locales are described briefly in a paragraph or two. Most locales are meant to serve as a springboard for the referee to create his own details. In addition, geographical entities, organizations, and realms that span multiple hexes are described as well.
description from designer's blog:
Blackmarsh is a complete, ready to run setting for your campaign. It can be run as its own setting or an expansion of your existing world. Contained in Blackmarsh are 17 geographical entries, 78 described locales, and one detailed town; Castle Blackmarsh. Each entry provide one or more adventure hooks to use in your campaigns. The text and map for Blackmarsh have been released under the Open Game License for anybody to use for commerical and non-commerical purposes.
This setting is intended as a setting for the Delving Deeper but is easily used with any version of the original D&D rules or retroclones.