Thank you for reaching out with this question! I understand you want to manage tomato blight while reducing risk to your honey bees.

Bee precaution pesticide ratings / University of California Statewide Integrated Pest Management Program (UC IPM) (ucanr.edu) allows users to search pesticides by trade or common name to determine the pesticide's known risk to bees. Copper sulfate has a rating of "III No bee precaution, except when required by the pesticide label or regulations."

It is critical (and the law) that pesticides area applied according to the instructions on their labels. The Protect Pollinators: Read Pesticide Labels card describes where on a pesticide label you may find information pertaining to protecting pollinators.

Even pesticides that aren't acutely toxic to adult honey bees can still have unknown sublethal effects, so it's best to reduce pesticide exposure to bees. You can apply pesticides at night after bees are foraging to allow the pesticide to break down for several hours before bees come into contact with it. You can also reduce exposure by keeping pesticides off of blossoms and water.