Pilot Iroshizuku Tsuyu-Kusa fountain pen ink is a blue ink inspired by the Asiatic Dayflower. It's a strong blue and it looks like the most neutral blue of all the blue inks from Pilot Iroshizuku. You can also say that this is the darker version of the light blue Ama Iro.
This ink has a good contrast against paper.
For writing and drawing purposes, this ink performs well. There's an attractive sheen, a gradation of colour, to the strokes.
I think it's a bit bright for writing, but I can see that it's really quite a pure blue so you really can't go wrong with this if you want a true blue ink.
Below are direct links to the ink on Amazon.