I've seen the question posed several times as to what a "mature" eggplant that is ready for seedsaving looks like, so I thought I'd post a picture of one.
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This is Kashalot, which is a large, dark purple-black variety. It can take a long time for eggplant to turn a golden or yellow-brown, so be patient. They will also be somewhat soft to the touch at this point.
There might be other ways to save eggplant seed, but what works for me is to cube the mature ripe eggplant up (I peel first), throw in a blender with lots of water, and blend away until I have a slurry.
Then I pour some of the mixture into a large cottage cheese container, add more water, wait a few seconds for seeds to settle, then carefully pour off anything non-seed. I keep adding more water, and carefully pouring off the pulp until seeds are clean. Just like tomato seeds, I dump eggplant seeds onto a noncoated paper plate and allow to dry a couple of weeks before storing.
No, a blender will not cut or damage the seeds (because they are so small), this method can also be used for tomatillos or ground cherries.