Blue is Krishna's special and significant colour, the colour of his aura when he manifests -- that is why he is called Nila Krishna. The adjective does not mean that he was blue or dark in his physical body. The whitish blue light is known as Krishna's light, also as Sri Aurobindo's light.
Krishna is not the supramental Light. The descent of Krishna would mean the descent of the overmind Godhead preparing, though not itself actually, the descent of supermind and Ananda. Krishna is Anandamaya; he supports the evolution through the overmind leading it towards the Ananda.
In this yoga all depends on whether one can open to the Influence or not. If there is a sincerity in the aspiration and a patient will to arrive at the higher consciousness in spite of all obstacles, then by opening in one form or another is sure to come. But it may take a long or short time according to the prepared or unprepared condition of the mind, heart and body; so if one has not the necessary patience, the effort may be abandoned owing to the difficulty of the beginning. There is no method in this yoga except to concentrate, preferably in the heart, and call the presence and power of the Mother to take up the being and by the workings of her force transform the consciousness; one can concentrate also in the head or between the eyebrows, but for many this is too difficult opening.