Such a cute video!
I have noticed, through having MANY kittens over the past 10 years or so, that those black kitties with the little white spots on their neck/chest and/or bladder is pretty common. Maybe 1/3 of our cats (keep in mind, we used to have like a FARM of them. Have maybe had about 70+ cats in my lifetime) were black, and 3/4 of them had a white patch on them in those areas.
Either way, those are some healthy, super cute kittens.
You can help her stimulate her milk by massaging her breasts every once in a while. One of my cats didn't have milk her first litter either. We learned the hard way that raising them to only formula milk makes them grow up malnourished and turn out to be rude, not so sweet cats

so if she's able to get her milk flowing, it'll really help!!