i did a small test to see which one does more damage. i made a lvl 1 fire corr and fire blaster. i only attacked lvl 1 minions and killed 10 on each char. i added up the total damage each one did. the corr and blaster where both using fire blast and only that power. both where hitting for 8 damage on the 1st attack. the blaster was hitting for 9 damage on the 2nd attack. the corr seemed to proc the fire dot more often than the blaster and scourge was hitting for 16 damage. this test was also with no buffs from outside sources, or reds, only using the base damage of the attack.
the blasters total damage on 10 different lvl 1 minions was 229 damage.
the corr total damage done on 10 different lvl 1 minions was 322 damage.
so overall corr do quite a bit more damage than blasters. this doesnt account for buffs or debuffs that each class can give themselves. just the base of the attacks.