Try using acetone to clean the surface to get rid of any oils.
Hard to tell the tone from those pictures, but if it's a white-silver then it's actually aluminum and the magnet was sticking to something inside the gun that's ferrous.
Stainless is very difficult to shape and stamp, and expensive, so very unlikely for an airsoft gun to be using it.
Stainless steel is more like a bathroom mirror sort of tone. Mild steel is a shiny grey or dull grey if you've sanded it.
Pot metals tend to be white/grey, and aluminum usually white/silver.
If it's steel, obviously a magnet will stick and the blue will work. If it's stainless, just put a light to one of the corners. If a clean oil-free surface doesn't blue, and it turns yellow, brown, then blue under a flame, it's probably a chromium steel. But the more chromium that's in it, the hotter it needs to be to change colors.
https://s-media-cache-ak0.pinimg.com...3f79a66b8b.jpg
Aluminum doesn't weaken until well above 500c so don't worry about that. Only problem would be if it's a leaded pot-metal. And as you get close to the melting point of aluminum, the surface changes to a very dull-grey, almost like it's sweating.