I would not consider The Black Stallion to be realistic or naturalistic fiction. In style and structure, Farley only escapes pure escape fiction by balancing the adventure story with verisimilitude.
To have a young boy be the sole survivor of a sea wreck is a bit much. A swimming horse? To have the boy survive the swim and island stranding is also a stretch. And for a boy to compete in a dangerous horse race and beat a seasoned veteran is not, by definition, realism. Also, the use of superlatives describing the horse gives it mythic, not realistic, qualities.
However, Enotes does suggest two ways in which Farley includes aspects of realism to temper the romance and myth:
1. Style (point of view)
The narrative is structured so that exotic features are balanced by settings and details that are commonplace. The novel features dramatic adventures. Alec experiences a shipwreck, a swim for dear life, an inhospitable island, a series of rides astride an unruly stallion...The stallion enters the novel in the midst of action made more effective by third person description from Alec's point of view.
2. Use of details in description
Farley employs contrast in settings as well, which include exotic and mundane locales. He balances the Black's extraordinary characteristics by portraying him with unremarkable elements of tack. Details related to racing lend verisimilitude, such as Henry's memorabilia of silver cups, newspaper clippings, jockey's clothes and cap. Slower-paced episodes balance dramatic scenes, although Farley never loses sight of elements that generate mystery and excitement.