I use both black and milk chocolate for a number of reasons: I find that the black chocolate tempers the richness of some of the cheese, while the milk chocolate provides a sweet balance to the sharpness of the other cheese varieties. And, also, I use both for the simplest reason that using two kinds of chocolate expands the variety of items in my platter.

I freeze my grapes for three to four hours, but I think there really isn't any hard and fast rule here. I like my grapes cold but not rock hard, their juiciness retained, and my guests not cringing from sensitive teeth. :P