I am not sayin' its aliens , but....a friend of mine called me on the phone a couple weeks back and asked me about this issue o his outdoor plants
he described the same thing... strangely in the same way... black sharpie.. wont wipe off... doesn't have any raised bumps..
I think his sounded more wide spread
I told him I had no idea what it was, but I am curious si im going to keep watch and see if you ever figure it out