It's a black ish tar looking substance and it smells bad, I have also seen where the hoof starts to get white and crumbly on the underneath side. I use apple cider viniger and water 50/50 and it works within days, you just clean the hoof out regular and squeeze or spray (i find a spray bottle works better) all over until its dripping out of the hoof. Don't worry, it shouldn't hurt her at all! Just treat it with something once a day.
It would be like a black ish green ish stuff in the little groove next to the frog and it will smell really bad. They usually get thrush if the ground goes from wet to dry or wet. If you are concerned she has thrush you could put some thrush buster on where the thrush would be to prevent or treat it. Hope this helps!