Sounds like rot!Hard to see in the pic.Cutting it out is always best and some of my books suggest unpotting and letting the roots dry and checking the roots for rot as well.Maybe your overwatering all your cactus at the moment?I'm sure some of the pro's will chime in.Check this link out.Doesn't have to be a saguaro.

Here is a link that might be useful: rot handling

I know this may sound weird, but it could be snails doing the damage. I picked up this notocactus at lowe's and it had a few black spots just like yours on it, and slime trails. There was a snail crawling on it. If I'm correct, I think I see some slime on the cactus behind the big middle one? I'd check the soil, and all the plants, you may need to repot. Just pick the snails off when you see them. I'm not sure if the garden snail/slug killer they sell in stores is safe for cacti & succulents. Good luck!

Jen

thank you for your help!

It is some kind of rot. No snails, since I have it indoors.

It's been spreading from one plant to around 10 plants. some took only 2 days to develop dark black spots, from originally pure healthy plants.

This is a real nightmare.

I have ordered surgical instruments at our local pharmacy.

Morgan