Warhammer 40k fluff thread VI: They see me Ward'en, they haten

[Squark]

Is there a stated reason why the Black Dragons are still allowed other than that their geneseed somehow passes the purity standards? Or a definitive reason how it does so?

According to Lexicanum, while the Black Dragon's geneseed tithes do pass the purity tests, the inquisition suspect they're stealing it from other sources.