I had a college who would meticulously cut that black thing at the bottom of the apple and also quite a bit of actual apple flesh, like 1cm or even more around it. When I asked him why he cut so much, he said that the black thing is remnant of apple flower and when apple tree is in the bloom it is the most sensitive so they apply most pesticides then and the concentration is highest around that black part. By cutting it away with little bit of flesh he removes most of the pesticides that are in the apple. It didn't help, few years later he died of cancer.