What is the essential role of narrowband filters in imaging nebulae that appear blue in processed photographs?

They isolate specific emission lines, such as Oxygen III for blue-green, for later color mapping

Specialized narrowband filters are vital tools in deep-sky imaging used to separate light components that the eye cannot easily distinguish. For blue-toned objects, filters targeting specific emission lines are employed. The Oxygen III line, which emits light in the blue-green part of the spectrum, is often isolated this way. The resulting monochrome image containing only the Oxygen III data is then digitally mapped to the blue channel of the final color image, often resulting in an enhanced visual impact of inherent blue structures that might otherwise be missed or diluted.