There really is no way to fix this, sorry. It it were another substance that had been spilled; it might have been possible to redye the carpeting. But, bleach is so caustic that if you dye that area, some of the remaining bleach will always cause that spot to be lighter. How about putting a runner down the hallway to cover the spot? At least you won't have to look at it.
My other suggestion would be to put an inlay in the hallway. You would remove a good portion of the hallway carpet, leaving the outside along the wall. Pick a complementary carpet and inlay this in the area where the other carpeting was removed. Seam and tape the inlay piece. It would look like a rug with a border effect.
Problem with this is that it would be difficult to do yourself if you have no experience and without the right tools. And, this type of job would be too small for most carpet stores to bother with. But, I would still check into it. Take several photos and measure the area carefully. Print and take the photos to a local carpet store to see if they would be willing to handle this request. If they do, remember it is really important to choose a carpet for the inlay that is of very similar weight and height to your existing carpet. Do not try to match the color. Pick something that coordinates with the rest of your color scheme.