I had to use a left over razor from a previous cloning. No choice, I was out. It's not what I like to do, but it wouldn't have really effected me if I would have got a bad batch, although I didn't. I did get the black stuff on my razor as you did. I had no idea what it was, and really don't now. Just wanted to let ya know that all my clones where fine and I even used that razor a third time. I can't tell ya what it is, but I don't think it is harmful to your plants.
P.S. I did sterolize there razor! I wouldn't try to infect my ladies. lol.