I Have to say you also made a good point, I was about to accept that it couldn't be justified to have a pureblood inquisitor, If it had been stated that the entire bloodline was enslaved in Tulak Hoard's era then any pureblood traits would likely have been bred out due to the rarity of pureblood slaves, they would have had to breed with the more common Humans as you said, I think I read somwhere that it was roughly 2000 years between Tulak Hoard and the events of SWTOR, don't quote me on that though incase im wrong but by suggesting the inquisitor's family fell into slavery closer to SWTORs time I think it can work.