When Holly and Bluebell first locate Hazel and the others on Watership Down after escaping from the destruction of their home warren, Bluebell is first mentioned as: "Hazel recognized him as one of the rabbits who had come to arrest Bigwig, but he did not know his name." Later, Bigwig identifies him as Bluebell by saying in a familiar way, "Oh, it’s you, Bluebell."
All of this implies that Bluebell was a member of the Sandleford Owsla, along with Bigwig, Silver, and Holly, especially the statement that Bluebell had been one of the rabbits accompanying Holly when Holly went to arrest Bigwig.
Later, however, when Holly is telling the story of the destruction of the Sandleford, he talks about gathering together the few rabbits who had escaped the destruction, and he says, "...Bluebell here and Pimpernel and young Toadflax. Toadflax was the only member of the Owsla I’d seen...," clearly meaning Bluebell is not a member of the Owsla. Further, when Kehaar is introduced, Bigwig and Silver are seen together as they discover the strange Kehaar in the grass. The text says, "Bigwig and Silver had a good opinion of themselves. Apart from Holly, they were the only survivors of the Sandleford Owsla..." which also leaves out Bluebell.
Clearly, Adams had in mind Bluebell as just a rank-and-file rabbit from the Sandleford warren, but he slipped up when he first introduced him as one of the rabbits accompanying Holly to arrest Bigwig, implying at that point that he was a member of the Owlsa.