Definitely use some isopropyl alcohol on the blackness to disinfect. I see that many of the older growths are shriveled...is this recent? If so, it might mean that the roots aren't in the greatest shape due to rot. As for the black spot, clean it well with alcohol and then look at the extent of the damage. If it is rot, you may consider getting a recommended chemical to combat this, if it is a slug or other insect, perhaps slug bait and a spray of insecticide will protect it.
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