It’s related to the typical dropback of a QB. During the dropback, he is normally turned slightly toward his throwing side, so is less likely to see a defender coming from the other side.
A right handed QB is always going to have his back to the left side of side of the field, regardless of where he's throwing the ball. If he's throwing left, he may be angled slightly, so he'd have some shot of seeing the rush from the left side in his peripherals, but it's still considered the "blind side". Opposite applies for a lefty QB.
Example: He's throwing to the left side, but his back is still towards the left sideline. If number 97 on defense had been coming around the end instead of inside, he'd be hitting the QB's blindside.