Black Sapote are in the Ebony family, and White Sapote are in the same family as citrus -- the two are not graft compatible. In fact, plants of the Ebony family (including persimmons) have a notoriously low success rate with grafting.
For more information on fruits named sapote, see the chart on this document:
http://www.plantsthatproduce.com/col..._06_Sapote.pdf
and the photos I've been posting here:
Banana Gallery - Sapote-named Fruits
Note that Black Sapote fruit (D. digyna) is non-toxic only when ripe. A tree of the same name in spanish "Zapote Negro" (D. revoluta) is a deadly poison 100% of the time.