Just to clarify, you have to do the Naturalist Quest and search all FIVE hives in order to get a black or white seed -- these are the ONLY seeds that will produce any of the newer plants in black or white (hedges included), since you can't crossbreed a black or white plant, so there's no way to do the whole cactus-bug crossbreed thing.
However, when you do all five of the hives, you will get a black seed, a white seed, or a fire seed... so it's not even a guarantee as to what color you'll get.
I will say that with the black and white seeds, I do seem to have pretty good luck at getting the newer plant types, so much so that I question whether that's all they give is the new plant types. Still, it's going to be a lot of running and a lot of hoping and praying.