“A double blind study will only be useful if you can objectively measure the dependent variable—in this case, the level of pain. It won’t eliminate the placebo effect or the distraction effect. “
The dependent variable is objectively measured in this case by comparing it to the placebo. If recipients report a greater degree of pain reduction for marijuana than for the placebo, than there has objectively been a reduction in pain.
This isn't a discussion that allows for opinion. If the "recipients report" anything, then it is subjective by definition.