The sun-dried types are typically dark brown to black. I'm talking about what makes them different. The real difference is in the processing. They were of particular value in the Middle East. After all, the desert is a hot place.
Grapes dry out and give a nice tasty raisin.
The raisins would keep indefinitely without spoiling, making them a choice food among the wanderers of the day. Of course these grapes, and therefore the raisins, had seeds in them. The seedless type weren't a part of nature in those days. As a result, it takes between four and five pounds of grapes to make one pound of raisins. This process takes 2-3 weeks and produces the dark raisins that we are all familiar with.