Hi.
Ouch... poor duck...!!
Don't you know what caused this black sore?
...Look... I'm no vet, but if she was my duck, I would proceed by some steps...
▬ Day 1 :
Step 1 - Wash her leg with vinegar.
Step 2 - Put Cade Oil on her leg.
Step 3 - Wait until the oil dry to wash her leg with a mix of warm water and Apple Cider Vinegar.
Step 4 - Put her leg in a mix of warm water, honey, and Olive Oil - for 10-15 minutes at least -, while delicately rubbing the sore as much as possible.
▬ Days 2 to 4 - Each morning and evening :
Step 1 - Wash her leg with vinegar.
Step 2 - Put her leg in a mix of warm water, honey, and Olive Oil - for 10-15 minutes at least -, while delicately rubbing the sore as much as possible.
▬ Day 5 - Morning : (Redo Day 1)
Step 1 - Wash her leg with vinegar.
Step 2 - Put Cade Oil on her leg.
Step 3 - Wait until the oil dry to wash her leg with a mix of warm water and Apple Cider Vinegar.
Step 4 - Put her leg in a mix of warm water, honey, and Olive Oil - for 10-15 minutes at least -, while delicately rubbing the sore as much as possible.
▬ Day 5 - Evening + Day 6 to 11 - Each morning and evening :
Step 1 - Wash her leg with vinegar.
Step 2 - Put her leg in a mix of warm water, honey, and Olive Oil - for 10-15 minutes at least -, while delicately rubbing the sore as much as possible.
▬ Day 12 - Morning : (Redo Day 1 and Day 5)
Step 1 - Wash her leg with vinegar.
Step 2 - Put Cade Oil on her leg.
Step 3 - Wait until the oil dry to wash her leg with a mix of warm water and Apple Cider Vinegar.
Step 4 - Put her leg in a mix of warm water, honey, and Olive Oil - for 10-15 minutes at least -, while delicately rubbing the sore as much as possible.
...And...
...etc...!!
I would, in fact, keep going if I need to. Probably for a month or two at least...?
(If you have no Cade Oil - do you have (true and ready to use) Cade Oil in America? -, I would recommend you try this method, even without this product.
Honey and Olive Oil can make miracles... and vinegar can also help greatly!)
AND : if there is no result after a few weeks, I would then personally put a big layer of (ready to use) Green Clay on the duck's leg, and I would bandage the leg for at least 24 hours.
After these 24 hours, I would clean the leg with a mix of warm water, honey, and Olive Oil; and I would put again a big layer of Green Clay on the leg, and then bandage for 24 hours.
(...But really, you should not have to bother yourself with Green Clay if you follow the steps written above...)
...I'm not vet - I prefer to repeat myself -, but I'm used to employ these kinds of methods...
Good luck!!