They can cross-pollinate but it will not affect the mother plants and seedlings may be interesting but variable as the hybrids seem to be more popular than many of the species. With all the clematis I grow, I've yet to get any seedlings but doubt my summers are hot enough to ripen the seeds.

Blooms "all summer" may be a bit of a stretch, but group 3 clematis seem to have an extended bloom period that starts in late June here and continues until frost. These are often some of the easiest to grow, as they are less affected with clematis wilt than group 2 hybrids and have a very simple pruning regime - just cut 'em back hard in early spring.

If you want seedlings of your plants, make sure to collect the seeds when they are fully ripe and they have a dark brown color. Most people pick the seed much too early and are unable to get results because the seed was not ready. The best time to pick seed is very late in the season, as this is when it is certain to be ripe and completely brown. Only the big fat brown seeds are fertile.
Germination of the large flowered types takes about 1-2 years in a dark, cool place. I place the pots into plastic zip-lock bags and check them weekly beginning at about 6 months and there-after. (Sometimes, you can get early germinations). Patience is all that is required.
All the seedlings will be a bit different than their hybrid parents and sometimes the difference is actually quite amazing ! The seedlings will need to be raised indoors for one full year as they will likely die if planted outside at a tender age.
It's an enjoyable hobby to raise your own 'new' hybrids. It CAN be done with patience and some TLC.
Below are a few seedlings which I have grown. These were ready to 'plant out'. Many more are being raised inside as they are still too young. I'll set these outside when the weather warms and water them faithfully on a daily basis.
Suzy

Here is a link that might be useful: my seedlings