Steganography Decryption From Completely Black Image

I am trying to decrypt a completely black gif that I know has been encrypted by some sort of steganography. I've tried subtracting the bytes of the images with each other, and so far I have gotten a really weird image consisting of seemingly random patterns of black and white pixels. What are the most common techniques to go about decrypting this image?

You can retrieve the message by restoring the colour table in a graphics editor like Gimp (Colors → Map → Set Color Map) or by editing the data directly (change bytes 13, 14 and 15 of the file from 000000 to ffffff).