A sub-group of white hat hackers who are hired externally by companies to perform penetration testing and sniff out bugs and vulnerabilities before a big launch. They are similar to white hat hackers in the sense that they use their hacking knowledge to improve cybersecurity measures, not bypass them. But white hat hackers are usually part of the company, whereas blue hat hackers are outsourced.
Sometimes a hacker seeking revenge is also called a blue hat hacker. They are not after money or destruction for the sake of it, they simply want to avenge someone — a person, a former employer, or a whole country. They work similarly to black hat hackers, with criminal intent and using illegal methods, but their purpose is not the same.