Black Rot is a fungal infection. Silver has not been shown to be effective against fungi (or very many other things for that matter). I would worry about ingesting the silver (because it will get onto the tiny grapes) and whatever other compounds are associated with it.

Copper is the usual control for fungal diseases. It is about the only thing organic growers could use, but they may have other agents nowadays.

I cut down some of the grapes I had because they were so susceptible to BR fungus. I have since learned that sanitation of the grape area and choosing the right variety are the biggest weapons against BR. In other words, make sure all the mummy grapes and leaves are raked out and mowed over in the fall. The fungus overwinters on them.

Finally, if you have to use a massive assault to prevent your vines from defoliating or getting those ugly brown windows in them, look around for a list of grape varietals that will do better in your climate.