Blackwing isn't just magically bound, he's soul-bound to V. If V knew of this, it implies that he didn't care enough about the soul of his familiar while making a deal which would have damned it more or less temporarily to hell. I don't know what else it can imply, but, if I had to look for an explanation, I'd say there's a contract between V and BW, in which V gave BW more power than a common bird could have and BW accepted to use it in V's service. For the exchange to be made, the souls had to be bound. But if V had known this, would BW also have known?
Anyway, you can understand that BW is a magical raven by his yellow beak. That's where his magic lies hidden! It's a shared trait with Mr Poe, Magica de Spell's brother and familiar. Normal ravens have a black beak.