As a Blackshield, you forego your oaths to your Chapter and swear yourself to the Deathwatch for life. The exact reasons why do not need to be shared with the members of the Deathwatch: only that you are still willing to serve the Emperor to the best of your abilities. Blackshields are willing to take the worst jobs without complaint and are very willing to give their lives for their battle-brothers.
The thing is, Space Marines are not meant for things like political intrigue. And a Space Marine using the black market is all but unheard of: at best the entire thing goes against the spirit of the Deathwatch, at worst it is outright heresy. The conflict I see here is not with the oaths of a Blackshield, but instead with those of a Space Marine. You are REQUIRED as a Space Marine however to report laxity in duty to the resident Chaplain. As far as I know the Deathwatch are no hardliners on this sort of thing (though it also depends on the origin of the Chaplain in question), but they will not tolerate heresy.
Oh, and if I were your Space Marine I'd stay far, FAR away of your fellow Unforgiven (Dark Angels + successors), lest they brand you as a Fallen and hunt you down.
