Answer: It's sometimes difficult to identify insects without actually seeing them, but from your description I think it might be a Buckeye caterpillar, which turns into a beautiful butterfly. I found this description and photos in "National Audobon Society Field Guide to Insects and Spiders." The caterpillar can grow up to 2 inches, is "black-gray, striped yellow-orange or whitish along sides, and has many paired branching spines." You didn't mention the stripes along the sides, but in the book's photo, those stripes aren't really noticeable, but the spines sure are!
The butterfly is a creamy brown, with colorful "eyespots" and chevron markings of blue, white and red along the edges of the wings. The caterpillars feed on low herbs, in other words, your borage. Borage is usually a pretty vigorous grower. If you can tolerate a little chewing on some leaves, you'll be creating your own butterfly garden! I'm jealous!
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