The alternating phases of light and darkness, as symbolized by the sun and the moon, give this earth its life. Without darkness plants could not survive. Without light plants could not survive. With both, there is life. Man is the same. Without evil, man could not exist. Without goodness, man could not exist. With both, man is who he is. It is therefore dictated that light and darkness are in the blood as well as in the sky and it is that balance between the two which gives man life.
Now as to my point of view on this interpretation:
Whether the phases of light and dark in providing life to earth and its inhabitants that can be true for sunlight does effect life, i.e. sunlight gives us vitamin D, etc. Plants also require the sun to grow and flurish, etc.
As to man not existing with out evil I do have my doubts about this for I firmly believe that we all can survive just fine with out that yet when you look at the Universe there are always opposites, i.e. light/dark | high/low | good/bad | and so on...so if there is good then there has to be bad but the degree does not have to be extreme.
Now, as far as light and darkness in the blood well that one I have to take umbrage with because tho the cycles of the blood in our bodies can be likened to the cycles of the sun and moon metaphorically in reality it does not wash, i.e. light and darkness in the blood.
There is no light and dark in the sky but there is the cycles of the earth in relation to the sun and moon where depending on the rotation it can be either light or dark in the same place and lets not get into artificial light which we can get from the sun's reflection off the moon.
Anyway, I liked this take and wanted to present it here for your own analysis, etc.
Oh yea, lets not forget this interpretations reference to "symbolized" as that opens the door even wider as to your interpretation.
Kingwood Karate School - Isshinryu (body - mind - spirit)
Oh yea, as for me I firmly believe that if humanity were focused on remaining in the present moment that we would be in balance thus removing good and bad because we would find our balance between both so humanity would be neither good or evil...