You don't have just one voice. You can sing from your chest or from your head. You can sing hard (like Barbra Streisand) or you can sing soft, like Johnny Mathis. Those variations are just for starters. You don't have to have blues voice everyday.

When you sing with your sister, close your eyes, listen to each other, and make your voices cuddle. That's blending.

Your voice and ears can do things that the speech mechanisms in the brain cannot define or express. That's why it's hard to explain blending to a person. So close your eyes, and let your lizard brain do one of the things it does best.