You'll just have to add more white icing.
But instead of adding white to the blue start a new bowl of white and add a little of the blue icing at a time until you get the right color.
You can freeze any leftover of the other blue color until you need it.
I haven't tried this ... but would Wilton White-White color work to tone down the blue?
I used to use the Wilton White every now and then but it leaked once in my color drawer and I don't know what that stuff is made of because it was impossible to clean up.
It was like a greasy glue that just would'nt come up, there is still a residue left on the bottom of my drawer.
Ever since then I don't use it, if I can't even clean it up with 409 then I don't want to eat it!
I've noticed that when I lighten colored icing with the white white stuff it makes the icing fade later on........like hours after the cake is decorated. IF it's in any type of light whatsoever it will fade and NOT evenly either. do what CakeMommy suggested and get a fresh bowl of white icing and add small amounts of your already tinted blue icing to it, and remember after about 10min. the icing will darken a shade or so as well. So better off making it a shade lighter than you want the end result, but then again I'm sure you knew that (your cake in your avatar is beautiful by the way)
The White-White is great to make white icing a stark white, but it also aids in preventing a color from going darker or brighter than you want. It sort of stops the "curing" process. It also helps keep the darker colors from bleeding onto the lighter ones. I'm not a fan of the Wilton White-White, but do like the ChefMaster or Americolor.
I do agree with CakeMommy, start with a new bowl of white and add what you have mixed up already a bit at a time until you get the color desired.
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