The Heavy Blaster definitely has high flux usage. 720 flux/s is the flux usage by definition, and it's higher than ballistic mediums and almost every energy medium except for Ion Blaster (under burst conditions), as well as every single ballistic large weapon + most large energy weapons (Only beat out by Plasma Cannon, and the Autopulse Laser under burst conditions). Basically no frigates can comfortably wield one, DDs struggle with them, and only some Cruisers and above can manage operating them sustainably.
What you're comparing is flux-to-damage ratio, or flux efficiency. In this case the Blaster is 12% worse than Mjolnir with that metric at armor=0, and Mjolnir is about 4.3% worse than Blaster at 1000 Armor.
I do not think giving the Conquest 2 free vents for using the Heavy Blaster vs a Mjolnir is a good assumption. Who doesn't max out, or nearly max out, their flux vents on capitals? That's basically a given on my Caps, and often I attach the hullmod that further increases flux venting. In practice that 2 OP isn't going to go to flux vents, but flux capacitors or a weapon upgrade somewhere on the ship, or perhaps upgrading a hullmod to a slightly more expensive one.
So using the base 720 flux the flux-to-damage ratio of the Heavy Blaster becomes 1.44 (15.2% worse than Mjolnir) at 0 armor and 4.31 at 1000 armor (Basically a ~1% difference in favor of the Blaster).
Also, the Mjolnir is one of the less efficient large weapons in terms of flux-to-damage ratio, but that's because it also has an EMP component.
I do like the Heavy Blaster, especially on higher-end ships with plenty of flux dissipation. But that weapon's basically the poster-child of flux inefficiency.