Why does my knife leave grey streaks in the butter? Sometimes my flatware leaves grey streaks in butter.  I've only seen this with knives in cold butter.  They claim to be stainless steel which I would think wouldn't do this. If I recall they were the very best flatware Target had to offer ;) Is something leaching into my food?
The photo suggests you are using a knife that has serrations.  Dirt can get trapped between the serrations. This is not likely to be metal from the knife, but the dirt.