Like all great poetry it is written so that it may be interpreted in many ways. Obviously Black water is at one level the river Blackwater near where Jimmy is from... but I also imagine its life... perhaps swimming there as a child with childhood sweetheart, her a natural free spirit.. swimming through life with the grace of a dolphin. unlearned in philosophy/religion, un(a)shamed (Sex/ Catholic guilt.. not awkward and self-conscious not geek like... contrasting with Jimmy the geek.. who's ability to turn base words and notes into priceless art contrasts with his fear of entering the water/life... while he ponders his own death and his own life .. because he seeks to understand life and death, fearful of both... she has just beautiful natural lightness of being.

OK the final verse is a Religious reference clearly but I take it as ... Jimmy isn't alone in pondering life and death... and that for those that strike the right chord, what we do echos in eternity (as will Jesus's teaching and Jimmy's art) .. most of us are doomed to be forget-me-nots lost in the snow (born to blush unseen, wasting our sweetness on the desert air) but others become the Blue Rose.