I think on BLU, most of the classes are quite underpowered if not played correctly. The only exceptions I can think of are Xan, Sorceress, and AcidLead. I always have had an easy time playing those classes. The classes just seem to lack the ranged combat that RED has. RED just has loads of explosive, long range classes that just are able to decimate BLU, mainly because BLU is oriented to getting up close and personal, just giving RED the advantage.
I assume you're referring to Bo Jackson and E Honda compared to Rebel and Vein (their red counterparts). They're meant to have higher learning curves. As for Vein being too powerful, I have an idea for some balances. Less ammo on his minigun, and an alternate broadsword weapon when he's on his bike. I'm curious which Red classes you think are too powerful in particular though.
Good beginner characters on blue are: Meower, Sorceress, Xan, Maiden in Black, Killah Zillah, and Acidlead.
I think I've buffed Bo Jackson to a level where his melee hits feel satisfying. It's not too hard to string a mega tackle into 1-2 stiff arms and kill someone very quickly. If this still isn't enough, I can always just give him a +20 hp buff, similar to the health buffs that Dhalsim and Ash Williams got.
Honda is actually the strongest class in the game based on dps. EX slaps does 180 damage per second. So really the whole point of Honda is to build enough meter and stay alive so you can kill problem classes quickly. If people still think he's weak, I could always find a way to up his survivability, either through +defense while attacking or a +health buff. Personally, I think he's near unstoppable with a healer.