Sounds like a callous to me. I've never gotten anything like that on cuttings in water.

I would transfer asap to potting mix. Good luck.

Doesn't sound like it to me. Callus appears as a lumpy white swelling.

Doesn't sound like a callous to me either. A callous is usually a white group of swellings.

and this would be when to pot it up??

I look for white callous that sort of curls around the end of the stem. That will take several weeks sometimes, depending on the rose. Some callous sooner than others.

I find the best bet is to keep the cuttings in a bag until callous develops THEN stick them in the dirt (and cover with a jar.) Callous keeps the black rot (which is what I fear you have) from developing. I have had stuff root after rotting some, but it does NOT HELP I am sure.

I read somewhere that if you root rose cuttings at a very high temperature (+70-80 degrees) that they would have a tendency to develop a callus instead of rooting. Don't hold me to it, because I got this off the internet and don't know myself if it's true or not.

The callus is the first stage of rooting.Then the roots form. What is happening with these cuttings is rotting.

Rotting is what I was thinking too..if my cuttings start to turn black on the ends, I pitch them, and try some more.