Can Blood-Kin steal freely from Orc strongholds
It's not about being Blood-Kin but about proving yourself stronger then the chief.
To be able to take what you want you need to talk to the chieftain of the hold and challenge him to a brawl (fist fight) with a 100gold wager. If you lose you lose 100 gold, if you win you get 100 gold and the permission to take items from the hold.
This has to be done for every hold separately but can't be done for Largashbur.
(Blood-Kin does give you access to the holds so you need to do that first to be able to challenge the chieftains.)