there are so many shades of blue and if you go to dark it could be a country blue, or sky blue, lol...the list goes on. I know what you are talking about. My sister is buying her first house and I asked her if she wanted anything in particular. She said she couldn't think of anything. She said everything they own is tan/beige and just to make it go with that. That is much easier than "blue".
I've seen my my sisters house once before, and everything she owns is plain. I can do just about anything as long as it isn't countryish(that's my new word for the day). I know you want to make something that goes and matches what she has. Is there anyway you can go visit or get a pic of what she has already? You can then pick fabric to go with it.