I see the pictures now. I see what you mean. It looks like microorganisms of some sort have taken hold there. This happens sometimes when I'm rooting cuttings. If the discolored area feels soft to the touch, or the cutting feels really deflated overall, you probably should toss it. Otherwise continue to observe and provide good care. If they manage to root and start growing, they will generate healthy new growth, and this may just end up a scar at the bottom.
Speaking of care, this is what I would recommend. I'm assuming these are indoor plants. Strong light is important. These cuttings should be in a place where they can "see" the sun for hours a day with no curtain or blinds in the way. Up in the northlands that would usually mean a southerly orientation, at least during the darker months. The more natural light, the better indoors. Water well when the soil is dry at depth, which might take a week or two at this time of year if it's warm and bright.
There is generally a sort of race between rot and rooting, to see which will happen first with succulent cuttings. You can bias the conditions to favor rooting (strong light, a good watering cycle where the soil does not stay moist) but a lot of cuttings will eventually rot out if they go long enough without roots. Or at least that's how they fail when they don't make it. Just remember that the water in the soil can only leave through evaporation up until such time as there are functional roots.
I would not expect 100% success in rooting cuttings in that genus, but some plants are easier than others, and you may just be learning which ones are trickier.
