From everything I've seen on leg color, it's a bit wonky. Unexpected colors can crop up from time to time. I did run the sussex roo x Dominique hen combo through a genetics calculator just to see if it popped anything surprising. Mottling (including mille-fleur) is recessive, so any babies from that cross will be "split to mottled" meaning the coloring is hidden UNDER the main color. Sometimes it pops through in later adult moults like this
View attachment 3797985The 2 black crested hens (at the back next to the barred rock) are gold penciled hamburg x mottled houdan. Until their last adult moult, they were identical all black...except for 2 white feathers on 1 wing of the one that now has all the white patches. The last moult, she popped all of that.
Your black chick is definitely from your Dominiques....and sexlinked. 100% of pullets from that cross will be black split to mille-fleur (or speckled). They may pop some of the Sussex coloring as they age. 100% of cockerels from that will be barred (and may pop some of the red tones later).
Leg color:
https://www.backyardchickens.com/threads/leg-color-genetics.472912/post-5997315
Looks like yellow is recessive to white, so white leg birds can carry genes for yellow legs but not SHOW them. In order for the yellow to show up, both parents must carry the yellow and luck of the draw, pass it to the chicks. Based upon that info, there's a good possibility that all your Sussex carry the yellow leg gene. Recessive traits are difficult to breed out of a line being as they don't show but can go along for the ride.
If you're breeding for show, none of the yellow leg chicks will work, but any white leg will pass muster. Just bear in mind that they are likely carriers of the yellow leg trait. Punnet square math says that 25% of the offspring of those 2 birds will have yellow legs, 25% will be pure for white leg, and 50% will be white leg carrying yellow genetics. 75% of the babies will be white leg.....but roll the dice on every egg.
Edit: forgot to mention the cockerel will be single barred, so look just like the Dominique hens. Purebred roosters are double barred do look like they're white with black stripes. Barring in on the Z chromosome and hens have only 1 of those.