These are some of the holy grails of the US Master System library if you’re into variants. The blue label re-releases happened once SEGA got the rights back from Tonka (who was doing their US marketing). They re-released a few games with bolder titles, bigger graphics and genre starbursts on the cover to help the game stand out. They also released most of them with blue carts instead of the standard red. This site has an much more in depth article on it.
The seller says some of these are even harder to find than the US Barcode of Sonic, although since they are technically variants, the prices are significantly lower. He’s also listed a lot of normal SMS releases, which all look to be in great condition. Plus the demo kiosk carts of Fantasy
Zone, Gangster Town and Shooting Gallery.