Looks a lot like fin rot... is there a way for you to separate these fish into a quarantine tank and treat them with some anti-bacterial medicine?
That's really the only solution I can think of
Also, these fancies are generally pretty sensitive, they aren't very well suited to be kept in ponds.

Okay good! Let me know how it goes

Just for the future, if an area of a fish goes black after an injury, this is usually a sign of healing and the black will eventually fade away

It was a good call to get rid of the comets, single tailed fish such as the comet aren't always aggressive, but because they are very fast moving, it is usually best not to keep them with some of the fancier, slower moving varieties, not only because of the aggression but because their speed can stress out the fish.

If you treat the fish in quarantine as Nossie suggested though, I'm sure they will be fine, it sounds like they're injuries from the comets haven't healed. So do take up her suggestion of treating with anti bacterial.

As for the white areas? Do the white areas look fuzzy in any way? Or are they smooth?
And it is just localised to particular areas?

Let us know how you get on!