Hi Tigger,
Black centres in fancy cut diamonds (oval,marquise & pears)is referring to a bow tie affect which looks like the name, due to a variations in the pavilion angle causing a variation in light display a bow tie affect will occur across the width of the diamond unlike rounds having almost the same pavilion angle all the way around so they have the most even display of light. Bow ties are quite exceptable as long as they are not to dark and don't interfer with the beauty of the diamond.
CHEERS!