I'm no expert but it could be scaly mites? It's hard to see from the picture, on my computer...not a great screen..but that is one cause of swollen, red feet and legs.
If it is scaly mites, people say to either soak the legs in warm water and apply vasaline to their legs...you could probably do a search and get the exact directions...or you could try topical ivermectine I believe? That is supposed to work on the leg mites. From my understanding, it is a hard problem to get rid of and a coop cleaning is due, if it is mites.
Best of luck and hope you get to the bottom of the problem...hopefully another OP that has experience with this will pop on and give you some more ideas and help!
edited to add, this has been a really hard year for parasites with all the rain we have had on the east coast..I don't know where you are, but we have had all kinds of parasite problems this year especially with the very wet climate this summer.
One foot did have the black scab on the bottom. I don't know much about bumble foot, but I picked the scab off and sprayed it with the Manna Pro Scaly Mite spray to cleanse it. I sprayed his legs and feet tonight again. They don't look any different, but are cleaner.
He/she might have an infection if he had a scab. Search here for bumble foot. There are excellent instructions on how to get the infection out. Sometimes the chicken can take care of the infection by its self. I've had hens who had mildly swollen feet but didn't yet require 'surgery' . Their immune system kicked into high gear and the swelling went away. Eventually the scab falls off.