I'm a truth-telling Whitefoot, and ....
... therefore you still don't know which one I am.
I think they goofed on this one, because:
Both would have to say "I'm the truth teller".
Truther - must tell the truth - so he must say he's the Truther.
Liar - must lie - so he must say he's the Truther.
On a side note, neither could claim to be a lying Blackfoot.