It isn't that you "can't keep the seeds". Sure you can. No one will come to take them away from you. It is just that most would consider it a waste of time and effort to save the seeds to grow the following year since hybrids (if that is what you have) will not breed true, you will not get the same plant or fruit. That is true for any hybrid. You can grow it as an experiment if you wish but they will usually revert back to some form of their parent stock.

What the nature of that resulting fruit will be no one can say because it all depends on what the parent stock was. Many hybrids are crosses from multiple parent stock so it may be a different color, fruit size, shape, shape of the leaf, plant size, and taste. Saving and growing multiple seeds could result in multiple differences in the plants and fruits.

There is much information available, both here and on the net, about hybridization and its effects on subsequent generations. I linked some of the many discussions here about it below if you are interested.

Dave

PS: True Black Cherry (the variety) if that is what you have is open pollinated it is not a hybrid.

Here is a link that might be useful: saving hybrid seed discussions

Dave's explanation is very comprehensive.
But, you ARE going to get some kind of tomatoes, although it may or may not be the same as you collected seeds from. The parents from which they originated a hybrid had also been better tomatoes. So if you are not religiously picky about it, then it does not matter.

I also, suspect that the seed companies and hybridizers also want to sell seeds and have some revenues..haha

Thank you Dave for your explaination.Now I got it .
@seysonn,yeah,it will be fun to plant some F2 of my tomato next year to see what will happen.Maybe I will get something interesting.