Water or dye based inks are best for ink blending. For example, Distress Inks and Distress Oxides, Hero Hues, or various other stamp company lines blend evenly on smooth cardstock. You can blend with pigment inks, like Versafine CLAIR, but they don't dry quickly. So there's a larger margin for error (and fingerprints!). Solvent inks, like StazOn, are best for stamping images and words.