As far as I'm aware, rewards are not stored in the blockchain itself but calculated by steemd when it starts. You can access the reward data by calling get_content on a permlink.
Be warned: After 30 days the rshares and active_vote (and probably more) information is not calculated and thus not returned. This means you can only calculate rewards for post within that time frame. Also, pay attention to the mode, which tells you which payment has already been processed.
Ever wondered why posts older than 30 days seem to have zero votes? Now you know!