Usually, heist teams are made up of specialists: the safe-cracker, the gymnast, the mastermind, the werewolf, the hot one, etc. Hazel's team follows this trope pretty closely with Blackberry and Dandelion, who both have their specialties and who both use them to make sure everything goes okay.
The narrator makes it easy for us to know Blackberry's specialty: Blackberry is "the cleverest rabbit among them" (8.36), who is able to invent all sorts of things. For instance, he invents "the raft" (8) which is a full on revelation to these little guys. Plus he invents a way to break out the domestic rabbits from their hutch (25). And he invents part of the Efrafan raid plan, including the boat-riding part. From our point-of-view, it may be funny that Blackberry is a genius to the rabbits because he shows a basic (human) knowledge of science:
"Frith and Inlé!" said Dandelion. "They're sitting on the water! Why don't they sink?"
"They're sitting on the wood and the wood floats, can't you see?" said Blackberry. (8.41-2)
This may seem real simple to us, but to rabbits, this is straight up magic. When Blackberry explains that wood float and says "can't you see," we can read that as a jerky burn, as if he ended a sentence with "duh!" Or we can see it as a reminder that Blackberry just sees the world differently than other rabbits. Rabbits may live day-to-day, doing things the traditional way forever. But Blackberry is an inventor who remembers things (like the fact that wood floats), which can come in handy in the future. This guy's all about progress.
So when you put Blackberry into a new situation, he doesn't cry about how new it is. Instead, he says, "We're going to need some new ideas ourselves" (19.9). This is Blackberry in a nutshell: he's the guy with the new ideas.
By contrast, Dandelion is known as the speediest rabbit and the best storyteller. We don't have much to say about him running fast (other than, you know, good for him.) But we do have something to say about his storytelling habits.
The stories he tells are all traditional stories. The Blessing of Frith (6), the King's Lettuce (15), El-Ahrairah and the Black Rabbit of Inlé (31), and the Story of Rowsby Woof and the Fairy Wogdog (41)—all traditional. They sound like myths to Shmoop. This doesn't mean that Dandelion only tells old stories; in fact, we hear that he's working on stories of Hazel's adventures, which aren't old at all; and right before they trick the dog into following them, Dandelion notes to Hazel that, if this goes well, "we ought to have the makings of the best story ever" (45.35).
But Dandelion mostly tells old-fashioned stories and he tells them in an old-fashioned way. For instance, when he tells the story of Rowsby Woof (which is unfair to dogs), he includes a comment about camels. But no one there knows what a camel is. Not even Dandelion knows, but he explains that "it was in the story when I heard it…" (41.58). That's maybe Dandelion in a nutshell: he tells traditional stories the way he heard them, so that they express the traditional virtues, like cunning. He kicks it old school.
Now, in another book, you could expect Blackberry and Dandelion to get into a fight: You're too old-fashioned! You're too new-fashioned! Argle bargle!
But in this book, Blackberry and Dandelion work together great as a team. (That's totally how they end the book, the two of them helping Hazel to lure the dog from Nuthanger Farm.) There's no fight between them and their specialties work well together. Dandelion tells stories about trickery and Blackberry comes up with tricks. There's no real tension here between traditional ideas and new ideas because all these ideas are used for the benefit of the warren.