At the end of the Gold/Silver/Crystal Arc of the Pokémon Special manga, Professor Oak tells Gold how each trainer that he entrusted a Pokédex to has unique abilities that make their relationship with their Pokémon different from that of a normal trainer. Professor Oak labeled Blue a Pokémon Trainer because of his ability to care for and bring out the best in a Pokémon.
“Blue, he's good at training Pokémon, he's a Pokémon Trainer.” (Page 157, Chapter 178, Volume 14)
Blue has been around Pokémon since he was a young child. He was sent to the Johto Region by his grandfather to train under Chuck in order to become a better trainer. Chuck taught Blue that in order to be a good trainer he had to train his own body as well as his Pokémon. By working hard under Chuck Blue was able to learn the struggles that Pokémon go through when they fight and how hard they work to become stronger. His training under Chuck gave him an appreciation for Pokémon. Chuck's training made Blue realize that the only way to bring out the best in his Pokémon was to bring out the best in himself.
Through the years Blue continued training using the methods Chuck taught him when he was a child. He trains his Pokémon with an iron fist and makes the same effort in training himself. Blue is able to put himself in the line of fire because he has a faith in his team’s skills because he has gone through training right alongside them. Blue is far more in tune with his Pokémon and can understand the flow of battle. He is able to push his Pokémon to their limits because he’s faced the same challenges they have.