.50 BMG shooters - Question.

[BlasterBob]

Recently checked over a .50 BMG Amac single shot rifle and noticed on the ports of the muzzle brake, some light blue substance which may be some type of corrosion. The blue powdery substance was easily wiped off leaving absolutely no trace whatsoever to the metal. Anybody know if this blue deposit may have been from the primers (CCI) or the powder (U.S. Military pulled powder) or a combination of the two components? No big deal because it left absolutely no damage/marks on the metal but just wondering if any other shooters/reloaders of the .50 BMG cartridge have also noticed a similar situation. Thanks and as usual, will appreciate NO bull shit type suggestions.