Just as a note, the original boots worn on set where brown, dyed black. When you unzip them, you could still clearly see the brown on the back tab of the zipper as well as the dye all around the rim of the boot, so the brown boot is actually correct.
Here are a set of the brown boots I recently dyed black. Still need to add the extra flap over the zipper, but otherwise they are identical to the originals.
I used Fiebing's Leather shoe dye which does a great job but does rub off on hands and clothes a bit. I want to seal them but am afraid it will change the sheen which currently matches the originals pretty closely.