Not any particular card, but I think it was shocking for me when I first got into collecting boxing in the 1990's to discover that with very few exceptions (ie. Isaac Murphy), the only black athletes who were ever pictured on Trading cards in America in the late 19th Century and early 20th Century, were almost exclusively boxers.
As far as "shocking" baseball cards...I'm having a hard time thinking of anything relevant to me. The Billy Ripken card was more funny, then anything to me.
Maybe when I was a kid and I ran across the 1975 Zoilo Versalles MVP card with Willie Mays....and I loudly said to myself "WHO?"
Other then that, if you go into the Non-Sports card realm...there's some 1938 Gum Inc. "Horrors Of War" cards that are legitimately shocking.