Is the area in question soft? You may want to unpot and check the roots before you cut. It may just be a little discoloration.

If you unpot and you find rot then the area you have marked is fine for a cutting point;). The cactus cutting should have clean flesh no rot spots.

It's not soft. The markings have appeared almost overnight. I repotted it about 9 weeks ago into pumice (we have high humidity so its the safest method). Prior to this it was in dormancy for the winter (now summer). The roots were fine when I repotted and I waited 2 weeks to water incase of any root damage. I've tried to cover all my bases for potential problems :(

Tbh if it’s not soft leave it be and watch it like Rob has said. I have been guilty of prematurely cutting cactus because they look different one day, and come to find out nothing is wrong. From the two pics I‘m just not seeing anything that warrants a chop;). It could be the blue hue coming off that’s giving it that look. Keep your eye on it and if you notice major changes you know what to do.

Okay will take all the advice and keep watching. The leather glove sounds like a neat trick.

By the way Kara, the monkey tails definitely have some air roots happening if you remember my post a while back. Some are just hitting pumice level. All my monkeys are sprouting them now mainly on their undersides toward the base. I think it may be the fertilizer im using each watering and the fact that the pumice dries out pretty quick. So am experimenting with lower fert dosages and watering 2 x a week. Th weather here is getting hotter!