Seedless in Black Corianth grapes is due to

Seedless in Black Corianth grapes is due to Stimulative Parthenocarpy.
The stimulative type of parthenocarpy occurs when pollination occurs but fertilization doesn't. It occurs by the insertion of the ovipositor of a wasp into the ovary of a flower.
In the Black Corinth variety of grapes, pollination and fertilization take place but the embryo gets aborted subsequently resulting in seedlessness.