"Blue juice" is the name of the blue liquid used in chemical toilets, particularly in aircraft and "portaloo" style facilities. Generally, it is and should be used in any toilet system where the sewage is stored for subsequent disposal (as opposed to a normal toliet connected a sewage or septic tank system).
The primary purpose of blue juice is minimise the inherent sanitary and odour problems associated with storing raw sewage. Typically, blue juice is a sodium hydroxide (caustic soda) solution, which serves as a strong antiseptic.
The blue colour is the result of a dye added as safety measure, since bright blue tends not to be a colour associated with edible substances. This colouring is required by law in many jurisdictions, since blue juice (either with or without dissoved waste products) is poisonous.