A general purpose operating system like Debian can be the perfect solution for many different problems. Whether you want Debian to work for you in the classroom, as a games machine, or in the office, each problem area has its own unique needs and requires a different subset of packages tailored in a different way.
Debian Pure Blends provide support for special user interests. They implement a new approach to cover interests of specialised users, who might be children, lawyers, medical staff, visually impaired people, etc. Of late, several Debian Pure Blends have evolved. 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.
To clarify the relation between a Blend and a derivative which is frequently mixed up Ben Armstrong said in a discussion on the Blends mailing list: "While a Blend strives to mainstream with Debian, a derivative strives to differentiate from Debian."
Using the object oriented approach as an analogy, if Debian as a whole is an object, a Debian Pure Blend is an instance of this object that inherits all features while providing certain properties.
So the Debian project releases the Debian Distribution which includes several Blends. In contrast to this, there might be some other Debian related Projects, either external or non-official, which may create "derivative distributions". But these are not the responsibility of the Debian project.
A word of warning: The fact that a Blend covering a certain field of work does exist does not mean that it might be a complete drop in replacement of Free Software solutions for all tasks in this specific field. Some Blends just started to work on this and adopted the technical framework to formalise the work on the project but it might perfectly happen that there is just a lack of available Free Software solutions for certain tasks. Debian can do less about this because we just assemble a set of software which was developed outside the Debian GNU/Linux distribution. So it has to be checked whether a specific Blend is fit for the intended purpose, whether it might cover just some parts of a fields of work or whether it is just a concept to develop some solutions for the future.