We had some pretty good trade deals going on in the past which probably helped the absorption (outside of what Clan Blood Spirit did). The Burrocks did cooperate with Clan Star Adder for many years, assisting with colonizing planets, expeditions and the transfer of vital scientist materials (even with Clan Nova Cat, FWIW). Together, according to FM:CC, Clan Burrock and Clan Star Adder basically kept the profit and sold anything they didn't need to other Clans.
IMO, Clan Star Adder could have known about Burrock's Dark Caste dealings for a LONG time.