It sounds like the nipples are oozing a black/brownish substance and this is drying to form a crust.
This needs to be seen by a vet. Mastitis can cause this. Likewise breast cysts can cause the nipples to ooze (even benign ones) Certainly suspicious (possibly malignant)breast lumps can cause this.
If your dog has what is called "False Pregnancy" the changing, and out of balance hormone levels can cause oozing nipples. Also of course pregnancy, especially if an infection has got into the breasts.
Get her to a vet to have this checked out.
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