I'm not aware that M. carnea produces blind fruit as a regular thing. I know that M. crucigera does, as I got quite excited the first time I saw the fruit after flowering, but it happens all the time on that species. I don't have carnea fruit to check on, but it could well be a timing matter. I tend to harvest when the fruit is starting to dry up, as I have always believed that this most closely mimics nature, and nature would decide the time of highest viability.
Maybe that's a bit naïve, but I have failed with seed which I think was harvested too early.