In order for the blindside card to work does Boba Fett have to already be on the planet location or can I have him on the Mission, reveal blindside and start the mission with Boba?

I believe you can just have him on the mission I'm going to review it or is blind side of the action card if it's the action card it's an assignment and you play it before you assign the rest of the leaders

Action card assignments go off before regular Mission cards

So I need boba there first and then reveal a mission targeted in that system.

Blindside's card description says "Special - Use Before revealing this leader's mission"

Normally action cards require the leader to be on the board in a system, however this cannot be the case here as the leader would not then be assigned to a mission.

I think the wording is clear: You play this card as it says, before revealing the mission the leader is assigned to.