"if gender's on a spectrum, and colors are on a spectrum, and colors represent genders, does that mean bill nye was right?"
Blue (or, Blue Sans) is an alternate universe version of Sans that is completely blue.
No true further changes exsist beyond this, though Blue has stated he wasn't always colored this way, but hasn't elaborated on the fact (due to no one really caring if he's blue or not). Blue doesn't have any exstensive knowledge on timeline, or any special powers (except for being able to digest blueberries faster than most other Sanses). Blue possesses Blueblasters, which are blue-colored Gaster Blasters, and the ability to manipulate bones --- which are completely blue, making his attacks absolutely useless.
Blue has all the same relationships with the other Monsters, though, his comedy has a slightly larger range, encompassing jokes about colors, color blindness, and how: "if gender's on a spectrum, and colors are on a spectrum, and colors represent genders, does that mean bill nye was right?"
Blue is usually laid back, often sleeping on the job and taking breaks. Though Blue is usually agreeable, he becomes eerily serious at particular moments. He is also observant; Blue reads the protagonist's expressions and can often tell when they have already done certain tasks. Instead of liking ketchup, Blue enjoys the delicacy of blue food dye.
Blue, instead of bleeding ketchup, bleeds blueberry juice.
A bit of trivia about Blue is that, he was created for the soul purpose of seeing the reaction of fans of Sans.