Quite a few slurs seem to be limited to certain regions. In my experience, “spade” as a slur is not common in the south. It seems to be more prevalent in the northeast and upper midwest. TV’s Archie Bunker was one noted user.

wikipedia, “To call a spade a spade”

The phrase was introduced to English in 1542 in Nicolas Udall’s translation of Erasmus, Apophthegmes, that is to saie, prompte saiynges. First gathered by Erasmus:

Philippus aunswered, that the Macedonians wer feloes of no fyne witte in their termes but altogether grosse, clubbyshe, and rusticall, as they whiche had not the witte to calle a spade by any other name then a spade.

It is evident that the word spade refers to the instrument used to move earth, a very common tool. The same word was used in England and in Holland, Erasmus’ country of origin.
The Oxford English Dictionary records a more forceful variant, “to call a spade a bloody shovel”, attested since 1919.
The phrase predates the use of the word “spade” as an ethnic slur against African Americans, which was not recorded until 1928; however, in contemporary U.S. society, the idiom is often avoided due to potential confusion with the slur and/or confusion with playing card references such as “black as the ace of spades”.