I did an overall wash of phthalo blue, for starters.
Then I painted over that with a irregular wash of indanthrone blue. It still didn't do much for me.
Now, we're getting somewhere - not sure where yet ... I've done a w/w wash of a violet-black mixture, and I am lifting the shadowed leaves a bit. I'm sure I'll have to go back into the large leaves at the bottom for more depth and richness, but so far, I like the look of the deep dark.