The blue corrosion on a car battery or car battery terminal is copper sulfate that’s been exposed to some moisture.
The blue corrosion is formed on the battery and battery terminals when the copper in the terminal clamp reacts with the lead in the battery terminal and then it's helped along by the transmission of electrical current and sulfuric acid from inside the battery.
It's natural for it to happen and doesn't mean your battery is bad but you can use some baking soda and water to clean the blue or any other corrosion off your battery and battery terminals.
However it can also mean that your batter is possibly overcharging such as your alternator could be causing the battery to overcharged so I would get the alternator checked out just to make sure it's not overcharging which could damage the battery.