Answer: I found the answer! Your culprits are roseslugs. They aren't slugs at all -- they are sawfly larvae. The adults, which are wasplike, lay eggs between the leaf layers, and the larvae emerge to feed, leaving "windows" on the leaves. As they grow, they consume entire leaves. The larvae drop to the soil to pupate, and in some places there may be multiple generations during a summer.
Fortunately, your good care has kept your rosebush hearty. You can help it further by spraying a neem-based insecticide /repellent may repel the adults. If the tiny larvae do emerge and start eating, a dose of neem should alter their appetite. Insecticidal soap is another option.
If, despite your control of the roseslugs, the cane tips still blacken, then you may have rose midges feasting there. Tiny orange larvae emerge from eggs laid by tiny flies, and feast on the tender tips. Pruning is the best method of control, though perhaps the neem may repel them as well. Best of luck!
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