The reason it reflects green is because it is designed to go through photosynthesis using very specific wavelengths. Darkening a plant would only overheat it, as it would be absorbing wavelengths that it does not use.

Also the food coloring isn't going to dye the plant blue because the roots uptake molecules selectively and food dye is not something they are designed to uptake! The only reason youcan do that with cut flowers is that you are bypassing the roots entirely and introducing the dye directly into the phloem and xylem.

Think of your stomach lining; if you eat blue jelly beans, your skin does not turn blue... the dye is not absorbed as dye.