See my message in the thread at Low framerate when my line is fading away? - #11 by neilcsmith (which you were also involved in! 
) It’s doable but you need to access PGL and set the blend functions back to the old implementation of MULTIPLY. eg.
p.blendMode(MULTIPLY); // just make sure Processing does all the other blendMode related things
((PGraphicsOpenGL)p).pgl.blendFunc(PGL.ZERO, PGL.SRC_COLOR);