Blossom end rot is caused by uneven watering; have you been having weather that leaves the soil dried out, then downpours (or heavy watering) that soaks it? I am inclined to say "no" this doesn't look like end rot, but if your weather's been up and down rainwise, it might be. This "may" be buckeye rot, which is a fungal disease (google on it for pics and description). At any rate, I'd play it safe, remove the affected fruit and dispose of it (not in a compost heap; toss it in the trash). Keep a close eye on your other plants and don't handle the plants or fruit too much. You don't want to spread anything around. When watering, be careful to only water the soil, not the plants, and don't let water/mud splash up onto the plants. I wish I could be of more help!
