This looks like bluing salts "bleed out".
When a gun is blued it goes into a tank of neutralizer after the actual hot salts bluing chemical to stop the chemical reaction.
If the gun isn't boiled long enough all the salts residue won't be neutralized in tight areas like the barrel-receiver joint.
If the bluing chemical isn't fully neutralized, the residue will bleed out in the form of a white powder.
To stop this, apply a heavy coat of a penetrating type lubricant like CLP Breakfree to the area.
Use a clean tooth brush to scrub off the white powder, then apply more CLP and let soak for a few days to fully penetrate the joint and stop the bleed out.
To make sure, just keep a drop or two of CLP in the joint.