I bought both white and blue chalk when I first got my longarm, then I heard horror stories from others about not being able to get the blue out, so I've never used the blue. One of my longarm teachers said she does use the blue, but she cuts it with white to make it less 'potent'. Keep in mind that you can have different 'reactions' on different fabrics, so I would ALWAYS test it on a scrap of the actual fabrics before I used it on a quilt.
Andi