They're very peaceful tangs. The Hippo may boss him around a little, but only if the Hippo is bigger will it last for any length of time. I've never actually seen the Blochii get that large in captivity. I've only seen them get about the size of a large hippo.
They do eat a TON of algae, so if you have two tangs already, something to think about.
To me, they're boring tangs, but free fish are always nice.
If you don't have that many other fish in your tank, I'd say go for it, unless you're thinking of a different tang. FYI, they're REALLY HARD TO CATCH!