Yes blackberry leaves and clover is okay to feed. (It’s also on the BB veggie fruit list) But there’s a few things to watch out for. First, if other wild animals, like racooons (who can carry diseases that can be contracted by rabbits) have urinated on it (which is how the disease is contracted) then that can be a problem. You also have to be sure that they weren’t treated by anyone with pesticides or sprayed with anything.
It’s also not good for your bunnies digestive system to make drastic changes, so if you can at least buy a small amount of the normal veggies selection and introduce the other options in slowly that would be better for your bunnies. You can also see how your bunnies react to the clover and blackberry leaves.
I know that our local organic market gets tons of carrot top as left-overs, and they give the tops away for free. Not sure if you have the option, but it might be worth asking.