Consumer Reports just published a study of US dark chocolate confirming that pretty much all of it contains high levels of cadmium or lead.
The trees naturally take up cadmium out of the soil, and then it accumulates in the cacao beans. But the beans don't contain high levels of lead, that's contamination during processing. (The article doesn't give a source for the lead. Of course I suspect gasoline, since leaded gas is still used to power agricultural machinery, but really I have no idea.)