Black and royal-blue Montblanc inks are of the same type and can be used one after the other without concern. A fundamental cleaning is on a nearly new pen that contained a black cartridge only briefly not strictly necessary; a simple flushing through suffices. Recommended is the use of a converter for comfortable flushing with water until it stays clear. For cleaning, normal tap water cold or lukewarm suffices — due to short contact time no limescale settles. Important is never to mix Blue-Black (a chemical ink) with other inks, since it thickens in the process.