Yes, but not exactly as you describe it. There should be a published Mint fare for LGB-BOS that you can buy. Otherwise you're just buying LGB-SFO + SFO-BOS as separate fares and that is likely to be more expensive.
That said, the regular site often has trouble with mixed cabin itineraries like that. BlueInc is better at them. If you search LGB-BOS does it offer you the Mint fare option?