Mysterious spots on my new Katana, please help!!!!

Hi, recently I have bought my first Katana. I have used it for some backyard cutting. I have always cleaned it with a cloth (that was included with the katana) and afterwards put some Japanese sword oil on it. Even when I haven’t used the Katana I did the cleaning and oiling once a week since I got it (about a month now). Today during cleaning I discovered some black spots and lines on it.l (see picture). I can’t get them off no matter what I tried. Can anyone please tell me what that could be and how to get rid of it?!

Thanks in advance for your help and tips!