The blue pen disappears with water, but you have to be careful because heat will set it. Don't leave it in a hot place like a car until you have washed out marks. I have used it with no problem otherwise. I live in Arizona and do have to be careful with heat setting blue marks. It will take more than just a little spritz of water, you will need to douse it to get marks out.
I knew a hand quilter in OR and she used pencil and made light marks. My SIL uses childrens markers that wash out.