The OSC synthax that Resolume shows is:
/composition/layers/2/video/mixer/blendmode
OSC type tag: int 0 through 50
(actually it reads "trough", not through..

I tried it with TouchOSC, but it doesn't work. I tried a multitoggle (value range 0 to 50, exclusive mode, 50 toggles) and a multipush (value range from 0 to 49).
I tried single push buttons or toggle buttons, too, but no success. i have no idea what the value range should be for the single push or toggle buttons.
By the way - in the tutorial video I see three buttons per layer for blendmodes. In R6 it is only one (drop-down menue) button. I think the three buttons are very convenient and make a lot of sense, why was this changed?