My apologies for dredging up an old thread but just wanted to know what happened to your duck as we have recently purchased two Muscovy ducks and in the last couple of days one has developed a sore foot which has developed into a black spot similar to your second pic. Foot no longer seems sore but I'm worried, especially since another very small black spot has appeared on other foot. Bathing feet in Epsom and betadine separately, applying antiseptic cream and have started him on cephalexin orally today. (All that I could get my hands on in Australia without a vet visit).
Black spot started out as pale spot that was swollen and red around. Noticed today that his mate now has similar pale spot and very slight swelling so have started treating her too.