what makes barrel cactus skin wrinkle and turn black
the bottom 25% of the plant turns black and the spines at the bottom turn black also.
Certified GKH Gardening Expert
This is fungal infection. Usually, overwatering will lead to this, and after that will be the wrong type of soil. Black rots are usually fatal to cacti, but you can make an attempt at controlling this by mixing 1/4 3% peroxide to 3/4 water when you water it next. This can help kill off the infection to give the cactus a chance, but if it has progressed too far, then there will be no hope.
This article will help you with caring for these in the future: https://www.gardeningknowhow.com/ornamental/cacti-succulents/barrel-cactus/barrel-cactus-care.htm