I spread avacado on whole wheat bread instead of mayo and then put some turkey breast on it and it was delicious. It was so good that I won't use mayo anymore on my sandwiches. The problem is that after putting a tablespoon or two on the sandwich, that still leaves a lot of the avacado left. I don't want to waste it. How can I save it? I put it in the fridge but it turned blackish. Is it still good?