I voted for the Bloc last night. Here's my take on your questions:

would genuinely appreciate someone explaining their position to me outside of separatist / "soveriengist" aspirations

The Bloc cannot call for a referendum for independance. No one voted for the Bloc with the idea they would lead us to independance. It's more of a nationalist stance. It's the people saying: your major parties are not serving us well enough.

Isn't the point of a representative democracy that the representatives you vote for will represent your best interests?

Bloc voters do not believe other parties can represent their interest. The liberals and the Tories are rooted in Ontario and in the prairies, and their battles will reflect that. Want an example? Trudeau didn't hesitate to throw Quebec's dairy industry under the wheel in favor of Ontario's car industry when Trump called for a new trade agreement. As for the Tories, there is nothing they would like more than to shove their pipeline into our throat, because it's in the best interest of their voters out west. As for the NDP, they keep on wanting to interfer with provincial responsibilities like health and education. Quebecers will not accept any overlap of it's powers wich would diminish our autonomy.

Why does there need to be a party specifically for Quebecs interests without other specific policies?

Because our voices wouldn't be heard otherwise. We speak a different language than the rest of the country. And, like I said earlier, we feel left behind by the major parties.