What was the name of the show with the kid who turned into a blue monster?

The show you're referring to is likely "Big Hero 6: The Series." In this animated series, the character Hiro Hamada transforms into a blue monster-like figure when he uses his advanced technology, particularly with his creation Baymax and other gadgets. The series expands on the adventures of Hiro and his friends from the original "Big Hero 6" movie.