Certified GKH Gardening Expert
All bell peppers start out green. They must remain on the vine awhile to ripen into red. As for the black coloration on the bottom, this sounds like blossom rot, which is normally due to a calcium deficiency. You can discard smaller peppers with blossom rot but on larger ones, simply cut off. It is still safe to eat.
Here is more information: https://www.gardeningknowhow.com/edible/vegetables/pepper/how-to-grow-red-peppers.htm
Some bell peppers darken to a brownish or blackish hue before they turn red. If they are not rotting or softening in that area, they are just fine. Give them time and they will probably change further. Should not effect the taste if they mature with some coloration.