A Debian Pure Blend is a project completely inside of Debian aiming to provide an installation medium (e.g. a DVD) targeted at some certain user group. Such an installation medium is distinct from the Debian main or default installation mediums, which target a server or a desktop installation in very broad and general terms.
Contents
A Debian Pure Blend aims to cover interests of specialised users, who might be children, lawyers, medical staff, visually impaired people, certain academic fields, etc. The common goal of those is to make installation and administration of computers for their target users as easy as possible, and to serve in the role as the missing link between software developers and users well.
Idea
The ideas behind Debian Pure Blends are
The Debian 8 "Jessie" release comprehends about 43,000 software packages. Without knowing a certain software by name, it is highly unlikely to stumble over it by pure chance, especially due to the odd naming.
Technical
GNU/Linux distribution that is configured to support a particular target group out-of-the-box. All changes and improvements are integrated to the official Debian repositories. A Debian Pure Blend can contain multiple flavors (or profiles) (e.g., Debian Edu has flavors for main-server, workstation, and thin-client-server).
Technically a Debian Pure Blend builds a set of metapackages and provides an overview about the packages which are included and which are on the todo list for further inclusion. Both pages are rendered from the information inside the tasks files in an SVN.
Most of the distributions based on Debian, like for example Knoppix or Sacix, are not Debian Pure Blends, and Ubuntu is not even binary compatible with Debian. Linux Mint Debian Edition is binary compatible with Debian, but is also not a Debian Pure Blend.