I have found abstracts to several articles where they say freeze the plant and pulverize it to extract growth substances.

And Fritz Went used "blue" light in some of his experiments but I cannot find any specifications. Do you know what he meant by "blue" light?

Sounds like the liquid nitrogen is just used to help reduce the plant tissue to fine particles to aid in extraction. When frozen the tissue will be brittle and easily pulverized using a mortar. The actual extraction would normally be done using an appropriate solvent.

When "blue" light is used experimentally it just means light in that part of the spectrum (430-460nm). For example, blue light has been shown to stimulate opening of stomata.