Both full back areas are well dodgy but at least Blanchard is defensively sound (reminds me a bit of Worrell) at RB even if he gets sucked inside too much, lacks pace and offers little offensively!
Purrington might well be the best full back at the club but we can't rely too much on a 17 year old.
The only alternative to both is Berry and as cover for both positions I suppose he does a job.