Wouldn't the White rice adsorb most of the water? this is one of the reasons I quit using white rice.
Never had a problem. You let the rice soak in water for least 2-3 hours or overnight before you cook. And depending on how much wild black rice you add to the mixture, you can make the final cooked rice really dark purple or just light lavender color. You can add all kinds of different grains, beans, peas, and nuts if you want. It's really up to your imagination and what type of grains you like. Most people tend to stick with 4-5 different grains but I've seen like 14 different grains mixed in along with chestnuts.
This video by Maangchi shows you how to cook multigrain rice using 4 different grains. She uses mixture of short grain white rice, brown rice, barley, and black rice but you can pretty much use whatever you want.
Multigrain rice is really common in Korean cuisine because so many people are health conscious. At many Korean restaurants, the server will ask you if you want white or multigrain rice with your meal. Here's picture of multigrain rice I had with mandu yukgaejang soup. They added beans to the rice which is also common. Mandu yukgaejang is spicy beef brisket and vegetable soup with added dumplings. Yukgaejang tastes very similar to Hungarian goulash soup. And that's hot barley tea in the clear glass cup. I know you have fascination with barley.