The seeds I picked are large and mostly black. Most of the green is not visible anymore. When I squeeze them to remove the pulp, the entire berry squashes. Can I dry them out to harden the seed or are they simply not ripe enough to be fertile?
Detailed instructions with images are here if you haven't seen it already:-
so if yours are simply squashing completely, it might be they weren't fully matured before you collected them, or you're squishing them too hard. They should have been completely black before collection, so I'm guessing it might have been you picked them too early. That might mean the seed isn't viable, or simply that it is, but you now can't peel them because they're too soft, so you're destroying them instead. I wouldn't try drying them out in an oven or something, but leaving them for a few days might mean they harden up - or might mean they just rot, not sure.