It's not logic, it's just the accepted definition of the term. "Blockbuster" most popularly refers to the highest grossing and/or highest budget properties. Bloodborne at 1 million units is what would be referred to as a "sleeper hit".
While I agree Bloodborne, or any Souls game for that matter, is no " blockbuster " calling it a sleeper hit is pretty laughable on its own merits.
Bloodborne could indeed eventually become a blockbuster. Even sales dropping by half the 2nd week+ it would still have the potential to sell another 500k worldwide, putting it close to 2,000,000 sales its first month. Then you have all the bundling that will be coming along with sales of the game and holiday promotions and DLC and probably an Expansion somewhere down the road.
Could become the best selling Miyazaki " Souls " family product yet. Somewhere in the realm of 4 million LTD would be my guess atm.
Demons Souls started out at 150,000 the launch week in the US and went on to sell 1.5-1.9 million WW LTD. Hows that for " legs ". Bloodborne could have the same type of legs with all the hype surrounding it. All those new players jumping in and getting hooked, telling their friends about it, those friends picking it up on a whim, some of them getting hooked. So on and so forth.
And really, to be perfectly honest, I guess you could call it a blockbuster. People think Final Fantasy is a blockbuster JRPG title and it sells around 1 million the first month in the US. Bloodborne could get close to that in the NA alone its first month.