Frank,
INK is bad for brushes. Anything you put on a brush--ink, paint, etc--is bad for brushes. That's why it's important to clean your brushes after you use them. I'm not aware of Black Magic being particularly bad for brushes, Frank, but I don't think that Black Magic is as good a quality of ink as it used to be. Black Magic USED to be THE ink of choice for decades, but sometime back in the early 90s, the company removed the animal fats used in the ink's contents. Many artists, professionals and amateurs, agree that the ink has never been the same since. For years now, I've switched to using HIGGINS SPEEDBALL SUPER BLACK India Ink. It's a fine ink, but it must be maintained. Every so often, you have to add a few drops of distilled water to it (5-10 drops should do it), and shake the contents of the bottle up to keep the ink flowing well. For my money, it's the best ink available right now. I've tried Pelikan, universal and all the rest, but that's the one I like best at the moment.
Buy a good brush and them when and after you use them. You'll be okay.
Loston
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