Blue is a young White Shepherd Gem Hybrid. He is the first known animal hybrid of a Gem to exist that isn't humanoid in nature.
Blue is an average size for his breed, standing about the same size as the Equestrian Ponies. His fur is mostly pure white, much like a standard white shepherd, with the exception of one paw and one ear, both of which are light blue. His gemstone is located on his forehead, and is in a vague shape of a sideways Diamond, similar to Pink Diamond, but right-side up. He also wears a hot pink collar around his neck.
Much like Lion, it is questionable if Blue is truly sentient or not, as at times he does behave as a self-aware being, and other times he behaves similar to a normal dog. He is a pretty playful creature, and has a good connection with his current owners.
Blue is found up on the side of the Crystal Temple cliff face, where he was promptly rescued by Lapis Lazuli. He was allowed to stay with the Gems, though at first gave them all a bit of a rough time. He does show his ability later on the following morning when he managed to track down, and dig up numerous Gem parts to build a communicator for them. As such, blue was allowed to stay.
In "Out of Dodge", Blue was left behind by Steven Universe, staying with Greg Universe. not wanting to stay, he got some help from Lion, and they both go off to try and catch up with him in Ocean Town. He managed to track Steven down all the way there, though sniffing out a part of the Gem Create first before finding the others.
When The Blue Light effected Beach City, Blue was the first to warn everyone when he found a ton of fish flooding the dock, barking to wake everyone up. He went off further away from the city, and found Starlight Glimmer, and quickly got the others to follow him back, only to find nothing there.
My Little Universe II
He briefly appears with Lion and Spinel trying to locate the remaining Harmony Gems inside the Supreme Kindergarten. It was his scent that ultimately finds them.
Blue has an extremely sensitive sense of smell, and is able to track down targets from miles away, shown when he tracked Steven's scent from Beach City to Ocean Town. He is also able to track down Gems, who apparently don't even have scents for animals to track.