Last week I found a tub of blue manic panic in my bathroom from when I had blue hair a few years back. There was only about a tablespoonsworth in there so I decided to dye just the "sideburns" of my hair. It didn't bleach well because red dye is so hard to get out, so I'm surprised the blue took as well as it did!
Whats really great is when my hair is down, you hardly notice the blue. So it's just a subtle change.