The Cross Fountain Pen Ink Cartridge Refill - Blue / Black. The Cross fountain pen will accept only specially designed Cross ink catridges or the bottled-ink converter included with each pen. The high-quality catridges contain inks formulated to provide both superior writing characteristics an complete chemical compatibility with the Cross fine writing instruments. It comes six in a pack. Available colors are Black (8921), Blue (8920) and Blue/Black (8924). Please see the fountain pen ink.