If you can get a hoof pick into the crack, you have a case of deep sulcus thrush. It is very painful for the horse and would explain her reluctance to move.
First take some Q-tips and get out as much of the gross black gooey stuff. The wash her foot with Dawn dish detergent. Scrub the area really well with a wire brush. Try to get every nook and cranny so to speak. Rinse well.
Next step is to apply something to address the infection itself. My personal favorite is Tomorrow Dry Cow. Tractor Supply sells it. If you can't get that stuff, the next best would be a 50/50 mix of Triple Antiboitic ointment and Athletes foot cream. Again use a Q-tip or syringe so you can get it into every crack. If the crack is really big, use a few cotton balls to stuff into the opening so it can stay exposed to air. This bacteria cannot live in air, so if you keep it exposed to air, that's half your battle. Keep doing this until the crack heals from the inside out and you can no longer get a hoof pick in there.
Last step, shop for a new farrier. A farrier who cannot spot thrush that severe has no business working on horse hooves.
You just have to see your distance...you don't have to like it.