What is the blue stuff?

Its blue dye,during the scene when she pours the box into the shower cap - if u can just about read the box(i just about read it myself) its says "dye" - my fav scene of the film

It is more than likely blue temper paint. It's a paint powder that you add a bit of water to to make it liquid paint. They use it in pre-k and kindergarten classrooms because it is washable and non staining. You can get it most any where including Wal-Mart. Just look in the coloring section, where all the color crayons are.

It's a package of Rit Dye. I recognize the color scheme of the box. That stuff will take forever to get off her skin (if it were the real thing and not what the Hollywood makeup people really put on her).

Benjamin put in blue Rit dye; although from the package, I expected it to be blood red...

Way late, but wanted to add, it dyed her skin blue. She then put on white makeup to cover it up, and regular makeup on top.