If it's still eating you can feed it until it stops. If it eats regularly and refuses food take the food out for a couple days. If it still doesn't molt try again. All of my Brachypelma slings start with a black or dark spot that progresses to the entire abdomen before they molt. The tend to have short 2-3 pre-molt periods at most when they are that small and short recoveries which get progressively longer as they mature. It's always exciting and just a little scary when molt time gets near. I've had 12 molts in the past 10 days and actually caught about 5 of them in the act which is always fascinating. Good luck with getting to watch.
When my B smithi was that size, and he got a shiney hiney, I stopped feeding him, no need to stress him out. He took about a week from complete black booty to molt. Just insure he has a water dish at all times.
Yup, definately looks like a molt should be coming up real soon. I would remove and prey items and maybe mist the side of the container a bit, don't soak it though. I wish you the best of luck for a successful molt. Molts are always exciting!