As a typical white person, I just don’t understand...is “boy” shorthand for “typical black person”?
No. One state to your southern border, it's used as a colorblind collegial term -- although it is meant in the strongest possible way when spoken with an edge.
Derived from the colloquial 'old boy' or 'dear boy' -- think late-period Victorian-era Arthur Conan Doyle British. Ky has imported much Anglophilia on the cheap -- idioms, manners, customs -- viz the Thoroughbred industry.