How to assess sunscreen effective via blueprinting?
From Wikipedia page on the blueprint process:


  The blueprint process is based on a photosensitive ferric compound. The best known is a process using ammonium ferric citrate and potassium ferricyanide. The paper is impregnated with a solution of ammonium ferric citrate and dried. When the paper is illuminated, a photoreaction turns the trivalent ferric iron into divalent ferrous iron. The image is then developed using a solution of potassium ferricyanide forming insoluble ferroferricyanide (Prussian blue or Turnbell's blue) with the divalent iron. Excess ammonium ferric citrate and potassium ferricyanide are then washed away. The process is also known as cyanotype.


Exposing blue print paper to UV light will cause it to develop blue. Sunscreen is meant to block UV light so applying sunscreen over blueprint paper then exposing to UV light and developing will give you a semi-quantitative way to evaluate the sunscreen based on the intensity of the blue.

See: Colorimetry