Black colouring on tip of fins

I have this small goldfish in a tank by itself for about 10 days. It
acts perfectly normal and happy. It looks normal too except that for
the past three days or so, I noticed that the top of the dorsal fin
has developed a very thin line of black colour. The tip of one of the
pectoral fins has a little black colour too. What is this? Do I have
to be alarmed by it? Is it an indication of disease?

The goldfish is the telescope eye type.

Thanks,
Petre