I think this is only a little bruise in the skin, it is most likely a result of your cat running into something during play.
It is not uncommon for this type of bruises to get darker during the first few days in the same way as bruises in people can do.
It does not look bad for now, but if it does not get better in a week you should take him to your vet.
The nose is an exposed part of your cat, so it gets easily bruised when cats play or fight their own reflection in windows or mirrors.
If there is any bleeding or discarge you need to take him to the vet.
