Mostly coined in Australia than anywhere else in the world, ‘bluey’ is (generally) used as an affectionate nickname for a redhead.
It is thought by some to have derived from the early 1900s as a form of irony. Blue is evidently contrasting with red, thus being used as a joke.
However, others believe that the source of this nickname came slightly earlier, around the mid-1800s. Irish immigrants arriving in Australia looking for labour gained a reputation as heavy drinkers and fighters, with ‘blue’ being local slang for a fight. The term evolved to come to mean a redheaded Irishman.
Of course, it could just mean “kind of blue”.