I did not water the cactus after repotting it (it's soil was very damp and was not drying out so I thought it might like dry soil for a week or so)
I took off as much of the old soil as I could without ripping the roots. I didn't think it was the correct soil (I bought it from a big box store). I put it in a cactus/succulent/ palm blend from miracle grow that had no water added to it and felt dry to the touch.
The pot is too large but it was the smallest my particular store had that was made of porous clay (6"diameter)
The soil is very dry that I moved it into. I hadn't gotten to water it yet.
The dark spot is squishy and I cannot rub it off. It moves under the outer "skin" of the cactus and seems to be progressing pretty fast
I have absolutely no experience whatsoever with grafting. I just went to it as a last ditch effort to save Sir Reginald (I named the stupid cactus). I have watched a dozen YouTube videos and lurked on a few forums today to try and learn and figured I'd try with one of the pups to see if a piece of Reggie could be spared. I know he can't grow on his own due to his lack of chlorophyll (I just learned this through Google)
I added the rocks in after because Reggie wasn't very stable and I didn't want him to take a tumble
Thank you for responding!
If I do graft I'd prefer to not do it on a tropical cactus again but I don't know what other species of rootstock I should consider
Also,it looks like the pictures being uploaded aren't the best quality... I'm not sure how to upload higher quality pictures