In a Harper’s Magazine review (Dec. 2014, pp. 84-6) of Allyson Hobbs, A Chosen Exile: A History of Racial Passing in American Life (Harvard) by Joshua Cohen, we read about the sad history of “black” people passing as “white”, with a story about the origin of the usage:
The term “passing” seems to come from the passes that slaves had to carry, which allowed them to visit their relatives on other plantations or when they were rented out for day labor.
Lexicographers and linguists will immediately smell a rat: the story is detailed and grounded in a very specific piece of history (and so is attractive to many people). But it’s only too specific: in fact, the usage is quite general, not restricted to blacks passing for whites, or to situations where some sort of pass is involved. Cohen’s account looks like an etymythology (aka mythetymology).