Although blue light is not as effective as red light, it is still quite helpful in encouraging chlorophyll production and plant growth. Combining red and blue light can produce exceptionally healthy plants. In general, plants prefer a good deal of exposure to blue light because it encourages stomatal openings, which let more CO2 into the leaves, thus strengthening the stems and leaves.
In short, blue light is vital for seedlings and young plants to create a robust root and stem in vegetative phase.
