I was reading your posts and replies from the various people. As a RN that loves wounds (I know, that's gross) I can relate the pictures that I am viewing to the chronic wounds I have seen in humans.
When a severe injury to a distal extremity happens such as frost bite, the degree to which the tissue was damage often leads to reoccurring problems. Just like the one you are having with a small black area on the limb that progressively become larger.
The recommendation of soaking in Epsom Salts was an excellent choice. The infusion of sodium to the necrotic eschar black tissue will naturally assist with the debridement of the hard Eschar tissue until it falls off or you remove it. Underneath will show a yellowish/greenish, sticky pus like base.
The Eschar is black and leathery looking such as in your pictures with the edges pulling away. The pulling away edges is the tissue underneath trying to rid the black Eschar scab.
That necrotic tissue is formed when healthy tissue dies and becomes dehydrated. This is a typical result of local ischemia from unrelieved localized pressure that will compress the soft tissue between a surface and the underlying bone or bony prominence. This will lead to an ulcer.
The underlying ulcer may be to a depth you do not know unless measured none the less, if there is any blood after the removal of the black Eschar scab, it WILL heal.
It does take time because it has to heal from the base of the wound upward. (from the deep inside to the outside)
Treatment for human ulcers is to keep the wound base moist, (Not the surrounding tissue) prevent tissue dehydration thus preventing the necrotic tissue cycle. The recommended dressing for this wound on a human is a Hydro gel dressing. It is made up of a starch polymer and 96% water and/or Honey impregnated dressings that contain a medical grade honey that has evidence to encourage healing, moisture and has shown antimicrobial/anti-inflammatory properties.
Keeping the area clean and dry with the addition of keeping the wound base moist will create a favorable tissue granulation environment. If you can find a way to secure and keep an occlusive bandage on her foot that would be a good start. IDK if they sale chicken booties like the ones they have for dogs feet. But, this would be ideal for the ulcer to heal.
The rest of the foot and leg appear to be healthy and pink in color. Gently messaging her leg always from the most distal end toward her heart is best. You can use coconut oil on her foot stump and up her leg.
She would most likely get use to you messaging her legs and really like it.
This is my 2 cents from a Nurse that loves Wounds and Ostomys.
Good Luck, I think your on the right road but, just needed a little extra human wound expertise to get her healed up. Please keep us posted.