Certified GKH Gardening Expert
If it's rotting on the bottom of the fruit, this is blossom rot. Here is more information: https://www.gardeningknowhow.com/edible/vegetables/pepper/pepper-blossom-end-rot.htm
This link has several problems w/ bell peppers articles: http://www.google.com/cse?cx=012078378210706707791%3Af1h5n_k1r5e&ie=UTF-8&q=bell+peppers+rotting&oq=bell+peppers+rotting&gs_l=partner.3...374729.381438.0.3826188.8.131.52.184.108.40.2065.2048.0j13j0j1.14.0.gsnos%2Cn%3D13..0.0.6673j3595529j20..1ac.1.m7KDJICX87A#gsc.tab=0&gsc.q=bell%20peppers%20rotting&gsc.page=1
Hope one of them will have the answer you are looking for.