Perfectly normal. Letting the patina develop will actually help to prevent rusting, so I've never seen it as a problm, myself. It's rust-bluing in it's most basic form.
I think that the patina helps give a knife 'personality' It's what makes it unique & while I love looking at bright, shiny, new knives, a well used, nicely patinted blade just stirs something in my soul.
I always used to tell my customers that it was going to happen & how to clean it up if they wished, but in answer to your question; no, I never had a complaint about it.