It doesn't matter what you feed the frogs.. you cannot increase blue pigmentation or the blue pigment part of green.. What you may be able to do in theory is push the green more towards the yellow end of the spectrum by increasing the levels of sequestered yellow carotenoids that are being used to filter the reflected light...I believe that spirulina has long been thought to try and supplement blues and greens but to the best of my knowledge nothing has ever been proven.
Last year Ed posted on a thread about dart frogs skin and iridophores and such. I'm trying to find the thread without much luck so far.
The reason for this is because metallic pigments, and blue pigments are not due to a sequestered carotenoid pigment. They are due to reflection (and in some cases polarization) of light from crystalline structures known as iridophores.