I think Zoro couldn't use Shusui's powers because it is a Black Blade, you need to coat a Black Blade with CoC to be able to use its powers.. So Shusui was locked rare artefact for Zoro.. I think Shusui has a power of its own like Enma and Nidai but Zoro can't use Advanced CoC yet therefore he got Enma in exchange.. I think Wado Ichimonji has a power too that might activate only when with Enma or something..
I agree on the main topic, Black Blades are forged with Adv CoC coupled with Adv CoA probably..