We’ll just ignore the fact that “call a spade a spade” (16th century) is centuries older than the racist usage of spade (20th century), and the fact that “call a spade a spade” refers to the digging implement (from Anglo-Saxon spadu), which is a completely different word from spade the playing card suit (from spada, Italian for “sword”).
You’re being disingenuous. A word or expression can have wildly different meanings in different centuries or milieus. Sure, “bitch” means female dog–at the kennel. That doesn’t give you a free pass in the office. And while “niggardly” has, I’m sure, the most innocent of etymologies, the manager who famously used it with a black employee in the DC budget office a decade or so back should have known better. He sure does now. An expression’s modern connotation exists quite separately from however Christopher Marlowe or Oliver Goldsmith originally intended it.
It’s been at least 40 years since I’ve heard ‘spade’ in reference to black people; give it another couple of decades, and it’ll be like how almost nobody knows that ‘welshing’ on a bet/deal/whatever was once a slam at persons from Wales.
FWIW, while ‘spade’ was a slang person for black people in the 1960s and early 1970s, at least in that era it just seemed to be slang, without any derogatory connotations attached. (Maybe in earlier times it did have more of a derogatory implication; I wasn’t around then.)
But I agree, it’s irritating when someone perceives offence due to their own ignorance. I once saw a Facebook exchange where a woman got very angry when someone asked her if she was retired. It turns out she was confusing the words “retired” and “retarded”.
A phrase in North England (particularly said of straight-taling Yorkshire country folk) is “He doesn’t call a spade a spade, he calls it an effing shovel.” I’m probably pretty naive but I always associated it with the gardening tool.