If you want it black, you can use deglazer and I know the "professional" oil dye will cover. I've had to recover from almost the exact same situation. Just make sure you have the leather as clean as possilbe of the previous dye before you re-dye it.
I've got a ton of belt buckles around. Can you show me a pic of what your talking about? My heads kind of thick, Im not sure Im understanding what your looking for. If I have what you need it's your for the shipping.
Here's my first rifle sling. I wanted to thank Tina for the pattern inspiration :http://leatherworker.net/forum/index.php?showtopic=1692
I'll post the colored pic from home. I forgot to download it off the camera : )