J. Herbin Poussière de Lune Ink

I think a medium-to-darkish purple is a great replacement for just black ink for a splash of variety especially since blue is usually the alternative. I'm normally a huge fan of Pilot Iroshizuku inks but I find their purples to be a bit too bright and saturated and the Herbin purple is just muted enough to still be distinctly purple without being too striking.