Answer: Your description sounds like bacterial rot also called wet rot. "Desert Landscaping for Beginners" by Arizona Master Gardener Press says the following:
"Wet rot, caused by a bacteria, can affect the stem tissue of cacti. Initially there is a softening of the tissue, which may be slightly darker than the healthy tissue. Infected tissue may become black, brown, red, or orange, and continues to soften. A bleeding or oozing of a dark substance will follow. Treatment involves cutting away the affected tissue with a clean, sharp knife, down to healthy tissue (which resembles a raw potato). Rinse off the area with a 10 percent chlorine bleach solution (9 parts water to 1 part bleach). Allow the area to air dry and heal."
I would not repot at this point, as transplanting is stressful. Good luck!
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