"Instant Blue" solutions are available from many different suppliers under many different brand names. Those of us that know call it a "copper blue". They are all basically the same kind of copper sulfate solution that reacts with iron to turn the surface of an iron content metal a blueish color. They work well to color iron metal, but only last weeks or months, depending on use.
It's a trick used by unscrupulous sellers, it doesn't last long, but it look's good.
All ya have to do is "sniff" the barrel,, if it smells like copper or a handful of pennies,,you can bet it's a cover-up.