"blue"(English)=obscene. Why "green" in Spanish?

[Douglas G. Wilson]

>the Spanish dictionary cites the "obscene"
>(or some such) meaning from 1852?

Yes.

<<VERDE. adj. Color de las yerbas frescas .... // .... [multiple other
senses] .... // Libre, inmodesto, obsceno: se aplica a' cuentos, escritos
poesi'as etc. // .... [multiple other senses] .... // El que conserva
inclinaciones o' costumbres impropias de su edad o' de su estado; como,
viejo VERDE, viuda VERDE.>>

So here "verde" ("green") is the color of grass, and a bunch of other
things including "Loose, immodest, obscene: applied to stories, writings,
poems, etc.", and (separate sense) "One who maintains inclinations or
habits inappropriate to his age or status; as, [green old man], [green
widow]." [My own casual and dubious translation]

Essentially the same items are in the 1992 edition.

Here the "dirty/sprightly old man" and the "merry widow" do not fall under
the "obscene" sense, but I believe there might be a transitional sense
between "youthful" and "wanton"/"licentious" as I said above.

-- Doug Wilson