I don't think it's magic, and the calories don't somehow disappear because you put them in your coffee. Yes, it won't kick you out of ketosis, but it's still energy you take in.
I find black coffee is sometimes hard on my stomach without a little fat to cushion it, and butter is a nice alternative to cream when I'm trying to limit lactose/casein intake (too much dairy makes me bloaty). I don't call it bulletproof coffee, though. I call it "coffee with butter". I also like butter tea. I bought one of those little battery-operated whisk things that are meant for frothing milk, and that mixes it quite nicely without the fuss of the blender.