Don't know about the black and green bit representing anything ,poppies have a black centre and green stem anyway but the emblem was chosen because of the poppies that bloomed in the battlefields during WW1 the red petals of the poppy being a symbol for the bloodshed.
The individual colours don't represent anything, except the colours of actual poppies. The poppy is a symbol of rememberance because, as a flower that grows in disturbed earth all over Western Europe, early Napolionic and WWI battlefields were covered in blood red poppies growing around the bodies of fallen soldiers.
As said above, it is the representation of the red corn poppy which grows naturally with those colours. I think I did read something somewhere, a poem maybe, where the colours were likened to red: blood, green: flanders field and black: the colour of mourning.