Once in awhile I get feedback on this. I have experienced it myself. Most of my formulas contain copper sulfate. I experimented and discovered there are two major reasons for this. The first is applying too much solution. The second is electrochemical. If you are applying Rustblue in low humidity there is a strong potential it will electroplate if there is a static charge in the metal. This can happen if you have just sanded or blasted the metal. Even walking across a carpeted room can build up charge as most folks know! To minimize this, attach the steel to a steel or iron wire and ground to some metal in your home or shop before applying the blue.
Hope this helps,