Not sure if this is still an active forum, but my guess is that, considering the story was inspired from 'The Childe Roland to the Dark Tower Came,' and Blaine could be an allegory to the 'Devil's Steed' in the poem. Pink being a reference to the bloodied rusty mane and general red description of the horse in the poem. He just went King with it and twisted it into pink, or Blaine at one point may have been red and time has simply worn down the color.
As for the poem
"As for the grass, it grew as scant as hair
In leprosy; thin dry blades prick’d the mud
Which underneath look’d kneaded up with blood. 75
One stiff blind horse, his every bone a-stare,
Stood stupefied, however he came there:
Thrust out past service from the devil’s stud!
Alive? he might be dead for aught I know,
With that red, gaunt and collop’d neck a-strain, 80
And shut eyes underneath the rusty mane;
Seldom went such grotesqueness with such woe;
I never saw a brute I hated so;
He must be wicked to deserve such pain."
Maybe.