Hi there! To me that looks like either a dried old leaf bit or a new leaf claw starting - the plumeria has roots already, is that right? If the plumeria has roots you are fine, if it is a rooting cutting then stop watering it until it has several full leaves.

It does not look like rot to me, if it were it would be more on the whole tip area, not just a dot or two. I think you are just fine!

I agree. I was just flicking off dead young 'claws' that hadn't turned into leaves yet from my plumeria growing tips this afternoon. I've never had black tip before but yours doesn't look that way to me either.

-Robert

My black tips and I had many this past winter (lost six plants and had to amputate a few branches) did not look like that. So I think yours are just old dead baby leaves which have died but as soon as new ones grow those will fall off.