Blackroot is a danger to new cutting and can be caused by not using clean everything for taking and rooting cuttings. It is also a good idea to let the cut end callus over for about 5 days. This cuts down on the amount of moisture the cutting can get. yes they were getting moisture through the stems much like cut flowers. Cold and too much moisture will cause the stems to rot.

Read my post on my amazing discovery about zonal geranium cuttings in this forum. I put my cuttings on a heat mat and never lost 1 cutting. I Started them the last week of February and right now they are bushy plants and 1 even has a flower stalk

I have been growing Geraniums from cuttings for 30 years or more and I disagree with Faye Brawner. Sure some geraniums will root easily without rooting hormone but not all. In my years of doing this I have found there is a difference in rooting between varieties and even colors. It casts more to buy 1 Geranium than it does to buy rooting hormone that will do lots of cuttings so why take a chance.

One thing of note--Always take your cutting from just below a leaf node. All roots come from the place a leaf has grown. If there is no node below the ground you won't get roots. Look carefully at any flower with side branches and you'll see that they too come from a leaf node