blue from sheetrock holes?

[pdude]

I removed an MDF shelf on the sheetrock wall of the garage. I puttied the screw holes and that was fine. Then I saw several pinholes behind it spaced sort of evenly apart. I put putty on it and it appeared to come out with a blue stain that almost seemed like it came from the hole itself as it has streaks of blue in the putty. If I put another glob over it, it is white, but if I swipe it then there seems to be some blue remnants.

Is it coming from the hole? Any ideas?

 

[Leaflessshadetree]

Did someone snap a chalk line when they hung the shelf?

 

[southalabama]

I'd vote blue chalk.

But poltergeists been known to live in walls.