well u asked this b4 pretty much and i like L's answer to it...in case u didnt see:
Ichigo's original reiatsu colour is blue, but it's only blue when he's not in his Bankai, he's just stopped using his Shikai, that's why we dont see it so much. Also his aura was always black in his bankai, it's only the outlining of the aura that's changed, remember that before it's outlining was blue? well when hollow boy came along, it became red. Oh yea and when it's pouring from his body, he's only exerting the excessive power.
In other words, he's evolved from depending on the powers of a Shinigami to the powers of a Hollow, he hasnt finished mastering his bankai, but as long as he can use it, he can get stronger with it, the powers of a hollow? it really depends on the inner hollow, and by how his inner hollow looks and acts, i think as long as he has strong adversaries, the inner hollow will grant him more power, that's if he can control it.
Anyways, Ichigo's like one huge tank of Reiatsu, while he doesnt have the skill to use it all, he ends up having plenty reiatsu leftover, probably enough for 2 captains to continue fighting. WHat i'm saying is, if he becomes able to control it all, he will either outlast or outpower the opponent, outlast when he is able to use the power many times over without using too much, outpower is when he chooses to exert high amounts of power on an opponent to literally destroy them.
