Mixing white and black rice?

Hi, I saw a bento blog a while ago where onigiri were made by mixing black (thai?) rice and regular white sushi rice. Result was really nice-looking black-purple onigiri! I didn't bookmark the page so can't find it anymore...but I'd really like to try it, too. Has anyone tried mixing black and regular rice? Any opinions what kind/brand of black rice works best? I don't own rice cooker so I usually boil the rice or microwave it, but I think I'll just have to experiment with cooking times then.