Question

what part of the leaf turns blue black? what about the remaining part?

Answer 1

The part where the starch is present will turn blue black and the other part would remain in the same color. We know that when iodine is used to stain the leaf the part where the starch is present would turn blue black and other would remain the same in the old color. 

Answer 2

when we put iodine solution onto the leaf, the part where the starch is present changes into blue-black colour. the part where the iodine solution  is not put remains unchanged colour. even if the part where iodine is present does not change blue-black colour ,it means that , there is no presence of starch.
that means , in the presence of starch only, the leaf changes blue-black