Normally parts which are bruised turn black. You probably jammed your thumb somewhere. It is the blood inthe bruised tissue that makes it look black.
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The cuticle of your fingernail is the whitish crescent at the base of the nail - the region where the nail is formed. I cannot think of any reason why water would turn this, or your fingers black. There are medical conditions, including simple bruising, that could do so.
Only Nelson Mandela can answer this question definitively. However it seems unlikely that, given his life of sacrafice and fighting for black causes, he would reverse his course now.
To turn green black, add black.
Low pressure, wear (usually after about 100 gallons) or a defective tip (rare).
capillaries run into your fingers some end there but some turn around and go back down