I don't know about washing denim....but...my mother's family's plots are in a cemetary with many, many, elderberry bushes along the edges...and the birds love them...and they (the birds) would leave their "purple calling card" on my Dad's headstone...she used to wash with laundry soap and water, and bleach, and Lestoil..to no avail....a sweet, elderly gentlemen, witnessing her tears (at the young age of 44) walked over and said---"Don't cry Miss, just use this"..presenting an open bottle of Coke, "it works every time"...and it did! We had a bottle of Coke with us each Sunday when we went to Daddy's grave (and Gramma and Grampa's and Aunts Mary, Cora, Anna, Bertha, Uncles Fred, Adolf, Ernest, Arthur and on and on and on)