Blossom end rot.
Most commonly this is due to insufficient levels of calcium, which can be caused by too little available calcium in the soil, pH of the soil being out of range, or too much of other competing ions on the soil, amongst other issues.
Most often, this can be fixed with the addition of a little lime (i.e. calcium powder lime, not the fruit) added to the soil. Will take about 2 - 3 weeks to resolve if that's the issue.