I'm watching the Naruto spin off titled "Rock Lee and His Ninja Pals" and during episode one, Lee says:
Just think of me as your hero riding in on a black horse!
Tenten then slaps him away and exclaims:
It's a white horse, idiot! Black horses are for the bad guys!
To which Neji follows up with:
Now, I've done some research and Wikipedia makes a similar implication that white horses generally have a positive portrayal:
White horses have a special significance in the mythologies of cultures around the world. They are often associated with the sun chariot, with warrior-heroes, with fertility (in both mare and stallion manifestations), or with an end-of-time saviour, but other interpretations exist as well.
However, I can't find a reputable (to my knowledge) source of information on the common portrayal of black horses. I did find one article that mentions their totems being omens of death:
The black horse totem is an emblem of death and rebirth. Its purpose is to aid you in leaving unnecessary things behind and showing you new opportunities that await. For this, it is required to take a risk and have faith.
Additionally, searches related to horse color within the context of Naruto have yielded no results.
As requested in the comments, the timestamp for when this happens is 6 minutes and 27 seconds in.
Is there any significance to horse color in the Naruto world?
Perhaps we need a tag for the spin off?