A Debian Pure Blend (in short Blend if used in clearly Debian internal context) is a subset of Debian that is configured to support a particular target group out-of-the-box. The goal is to provide solutions for Special Interest Groups inside Debian. The concept was formerly known as Custom Debian Distributions.
For a very short introduction see the Wiki page.
A comprehensive overview is given in the article covering Debian Pure Blends.
People who are interested in development of Debian Pure Blends should visit the project page on Alioth.