First, here are the rules I'm citing from the PRD:
Firearms are further divided into one-handed, two-handed, and siege firearms. As the category's name implies, one-handed firearms need only one hand to wield and shoot. Two-handed firearms work best when you use two-hands while shooting them. Two-handed firearms can be shot with one hand at a –4 penalty on the attack roll. Siege weapons are typically mounted on some sort of platform, movable or otherwise, and have greater power but a much slower rate of fire—they're detailed in their own section.
Scatter Weapon Quality: A weapon with the scatter weapon quality can shoot two different types of ammunition. It can fire normal bullets that target one creature, or it can make a scattering shot, attacking all creatures within a cone. Cannons with the scatter weapon quality only fire grapeshot, unless their descriptions state otherwise. When a scatter weapon attacks all creatures within a cone, it makes a separate attack roll against each creature within the cone. Each attack roll takes a –2 penalty, and its attack damage cannot be modified by precision damage or damage-increasing feats such as Vital Strike. Effects that grant concealment, such as fog or smoke, or the blur, invisibility, or mirror image spells, do not foil a scatter attack. If any of the attack rolls threaten a critical, confirm the critical for that attack roll alone. A firearm that makes a scatter shot misfires only if all of the attack rolls made misfire. If a scatter weapon explodes on a misfire, it deals triple its damage to all creatures within the misfire radius.
Prerequisites: Dex 13, base attack bonus +1.
Benefit: You can choose to take a –1 penalty on all ranged attack rolls to gain a +2 bonus on all ranged damage rolls. When your base attack bonus reaches +4, and every +4 thereafter, the penalty increases by –1 and the bonus to damage increases by +2. You must choose to use this feat before making an attack roll and its effects last until your next turn. The bonus damage does not apply to touch attacks or effects that do not deal hit point damage.
The question is as stated above: Is blast shot the same as the scatter weapon quality?
Arguments against: Specific overrides general. Firearm siege weapons are still subject to all firearm rules except where overridden by firearm siege weapon rules. That means that any firearm siege weapons with the scatter quality are subject to its rules.
However, the cannon doesn't have the scatter quality. Instead it has a special ammunition that specifically calls out cannons as being able to use it.
Arguments for: It's probably RAI
Why is this relevant? Basically, can I use Deadly Aim when using a cannon loaded with blast shot?
Other interesting side note: A cannon only has to MISS every target using a blast shot to misfire (I assume at least one roll has to misfire). A blunderbuss has to MISFIRE on every target using scatter shot.