I have had this problem where scion was green and at times pushing while union looked black, I opened up the union, cut the graft out, and regrafted it at new spot, removed blackened part of scion out. as long as you see fresh green moist wood when you cut scion again, it has a slight change it might take. I have been successful doing so.
few scions were shivered up a bit, upon regrafting the scions got boost and came back to life. if scion is precious and cannot be easily found, I would recommend trying this as an option.