Blended spectra are spectra from two or more objects that are not resolved, resulting in a spectrum that is a combination of the two. The term is also used sometimes for multiple spectra from the same object blended.
Producers of blended spectra include optical doubles including many binary stars, stars along with their extra-solar planets. Blend analysis is the attempt to tease out the individual spectra and thus information about the individual objects. A primary goal is to establish that it is a blend, which can mean determining that an apparent single light source actually is a binary or optical double. As such, it is a means of determining if an apparent transiting planet is really a binary star. It is also useful for the analysis of the spectra of a star and its planet.
Software to perform such analysis includes Blendanal and Blender.