Okay, Ava thinking Olivia was just lying again does make it a lot more palatable.

The Frank/Blake thing was sooooo forced. Blah. I have to agree that they never showed Frank getting over Natalia.
In the scene by the railroad with Olivia telling Frank to go after Natalia again after her return (smacks Olivia) is when I think Frank finally realized that Olivia really did love Natalia. Now, I don't think he thought it could last, that Olivia wouldn't screw things up/cheat/abandon Natalia - but he did seem to get what Olivia felt was real and not a scam.
What we did not see him ever realize is that Natalia was never in love with him. Frank still thought that Olivia stole her from him and ruined his relationship, not that the relationship itself was just a result of Natalia's gay panic.
Which would totally feed into his delusion of thinking that 1)things will not last between Otalia and 2)he can still swoop in pick up where things left off with Natalia when that happens.

And Blake deserves better than to be the rebound girl, anyway. *smacks Frank again*