Ya know i don't know why nobody has thought of this, but it just occurred to me that seat belts are woven nylon....and RIT dye will dye nylon just like it's fabric...so you can dye your belts black with that stuff and it should be totally permanent aside from possible sun fading, but you'll get that with anything. I know they'll dye because in the RC car world people use RIT dye to dye their wheels different colors, and the wheels are nylon, so there's no reason you couldn't do the belts too.
You can thank me later
I wasn't bashing or anything, just stating the obvious. You're workin with what you've got and maybe it's a little rough but you're doing great 
