i know trichocereus gets black spots that scab over as a result of bacteria (typically from overwatering)..
some similar process would be my guess...
also, i would try getting ahold of the vendor who sold them to you to see if he knows what is up.
Something interesting:
I have a peruvianas with the said black infection. It was outside and we had unusually high humidity plus rain storms.
I'm a cigarette smoker. On aNother forum, I read about a traditional method of healing cacti cuts with tobacco. (I have not seen any published information on this, it was a peruvian shaman) So I had the idea to blow smoke onto the infection. After 3 or 4 days of blowing smoke onto the black spots, the infection died back almost completly and new growth started.
Cure? Coincidence? I do not know.
hcc
p.s. I was smoking American Spirit brand cigarettes, the blue pack with the filters ripped off.
yes, I do need to quit smoking, even if its good for the cacti ;-)