Just to be clear: to be double-blind, both of you would need to not know which cubes are which until after the experiment.
If the experimenter knows which are which and the subject doesn't, it's only single-blind. The disadvantage of this is that the experimenter might inadvertently reveal some clue to the subject as to which is which. That would make the experiment's conclusions more suspect than double-blind.
If the experimenter knows which are which and the subject doesn't, it's only single-blind. The disadvantage of this is that the experimenter might inadvertently reveal some clue to the subject as to which is which. That would make the experiment's conclusions more suspect than double-blind.