I've just found Moina Michael's poem called "We shall keep the faith" that inspired Robert to write Bloodflowers' lyrics.
the main verse:
We cherish, too, the poppy red
That grows on fields where valor led
It seems to signal to the skies
That blood of heroes never dies
But lends a lustre to the red
Of the flower that blooms above the dead
In Flanders Fields.
the whole poem is widely available on the internet.