I'm 29 years old and have too many white hairs. After hair transplant, will I get natural black hairs again?

Grey or white hair is a genetic trait.  Transplanting hairs will not make your hairs black as the black donor hairs will also turn white.

Rarely I have seen transplanted hairs grow in a little darker in the initial first year or so, but they always resume the color they were before being transplanted.

if the extracted hair is grey, so it will grow in grey. If the extracted hair is black, so it will grow in black. Hair transplant means tranferring the hair grafts from your donor area to your bald areas, we have no influence on your hair colour.