I've grown cherry tomatoes, but never Black Cherry.
Some possibilities, in decreasing order that I would suspect:
- Old seed, or seed that was poorly stored (too warm and/or too much humidity)
- Too low temperature for germination (though if you have other tomato seeds that have germinated ok, this probably isn't the cause)
- You didn't wait long enough -- in my experience, some varieties take longer to germinate than others.
- Failed to keep the growing medium moist during germination.
- Kept the growing medium too wet during germination.
- You used soil from your garden and it has some kind of germination inhibitor. (E.g. winter rye was growing where you took the soil.) But you said "potting soil" so I'm assuming this is bagged soil from a store, so this is highly improbable, and I'm only listing it for completeness.
Things you could do to increase the odds of germination:
- Keep the soil temperature at 75°-80°F during germination. (A heat mat with thermostat would probably be the best bet. I also frequently hear putting them on top of your fridge as a recommendation. I don't do either, but I haven't really had germination problems except when I use old seed.)
- Use a sterile, soilless medium.
- This won't increase your germination percentage, but it will increase the likelihood that you will get a successful germination: plant more. Yes, this may waste seed, but if it isn't germinating anyway, what are you going to lose?
- If you're using seed that you have held over from a previous year, and you haven't stored it under ideal conditions, give up on this batch and either plant a different variety or buy some fresh seed.
If you do everything right, and you're still not getting a high percentage germinating, you should complain to your seed vendor and/or find a better vendor.