How about some more info?
HIGHLY doubtful that a slave would be in a white cemetery...that someone executed for witchcraft would be anywhere near a Christian cemetery.....
The other thing is, whites understood slaves to be full of African superstions and given to occult practices - take a look at Huck Finn - and a slave owner wouldn't execute a valuable slave (worth tens of thousands of dollars or more, in today's money) over something like that - there would have to be real significant harm attributed to the witchcraft, real malice seen - and then the slave wouldn't be buried in the owner's family plot, in a Christian grave.
Unless the owner believed the slave to be innocent, and the execution was against the owner's wishes?
Finally, executions of slaves were supposed to be legal, official and after a trial, however unfair. That doesn't mean, of course, that a white man would be prosecuted for unofficially killing his slave.
Your hunch that the lengend rings false sounds right to me.
Mrs VS
Does it really matter? It's a legend.