I found an article referencing Paul Harvey. It seems he is the one that made this remedy popular, and it looks like he recommended white raisins. FYI here is some info on types of raisins:
Dark raisins: These are the most common variety found in markets, usually made from Thompson seedless grapes. Although they start as green grapes, the fruit naturally darkens as it dries.
White or golden raisins: These are also called muscats and are generally made of white muscat grapes which are seeded, specially oven-dried (rather than by sun), and treated to retain their light color. Some golden raisins are dried Thompson seedless raisins which have been kept light in color