Those spots! I get those spots on some cacti. MS Smith (whose flickr account is something you want to see) has some pics of this too. Doesn't seem to be sickness. Does not directly lead to rot but may be an indicator of an issue that could.
The funny looking guy in the back to the left has the black stuff on the underside of the pup.
I think he had some suspected causes on a post on the nook, but It was all inconclusive.
I have a couple theories too based on what I gleaned from other posts and my own cacti....but nothing to back it up.
I'm fairly sure humidity and heat combined is factor. I had some cacti at a friends place that got this pretty bad starting near the base and working up 6-8 inches. I moved them to a new place, repotted them and while it never "healed". It totally stopped forming and didn't appear on any other cacti in that new location. Could have been the repotting, could have been the move. The 2nd location had better breeze.
Now, I've got everything in one yard, I'm here everyday. I've got a few zones... Full sun, shady, in between, side of the house blah blah. Trying to find the sweet spot, but I've got to leave them alone long enough to see what's what..... I will have to report back after a few seasons.
So, all is well
UNTIL , I've noticed now, on a 3 foot tall Torre Pedro that this black speckling is starting to show only near the top. I don't know, nothing else seems effected, and I've never seem this black stuff that high . My collection is all prettying young. Besides a bigger Cactus Kate PC, nothing is much taller the 3 feet. Most of the cool clones under my care are less than 2 ft and even my 8inchish seedlings aren't showing this black stuff in the same spot of the yard. It has been hot and rainy allot, so it may be that.
I'm keeping a watchfull eye, but it could even be some soil content, ph, fungus.... I don't know. It won't scrape off, so I believe its under the skin. I can't say for sure that I've pealed one with this, but when I think back, I truly must have. I do not recall seeing black under the skin. I think I would remember, so maybe it's inside the stoma of the waxy skin, like mold, mildew, fungus in the tiny pores.
No way in hell I'm butchering Torre's tip to examine the blackness, so that experiment will have to wait a while, at least till a base pup shows.