Conditional blending, also known as “blend if”, is a feature which is currently under development in our master branch. A general description of the idea together with some examples can be found here. In short, conditional blending allows you to limit the effect of a module to certain pixels of an image, determined by their color coordinates. For modules in Lab space, you can restrict the effect of a module depending on the pixel’s L, a, and b value. For modules in RGB space, you can restrict the effect of a module depending on color channels Red, Green, and Blue plus a Gray value.
This is a companion discussion topic for the original entry at https://www.darktable.org/2012/07/some-enhancements-to-conditional-blending/