Well, if it is Black spot, it usually is not a "killer" so little to worry about there. That is the good news!! The bad news is it does not seem like the things that people often mis-identify as black spot, melanin in cells etc.. Other bad news is: What is to worry about it the life cycle and the longer it has to breed the more complex the problem is to control. I would watch it for a day or two but take action soon. You know it is there, you know it is not going anywhere *unless it falls off to infect other fish and/or breed, so it needs to be addressed sooner then later. As a side note, if you catch it early enough, there have been reports of eradication without system wide treatment. I do not know how much I believe them but everyone knows I am the "treat first worry about it less" type of person.
I know it is going to be next to an act of god to get a picture of it close up but if you can it would go a long way... If not, try to look at it and paint a picture for me of what it is like. You can often see black spot moving on the fish if you can get close enough.
