There isn't a powerset i'd consider iconic for CoH.
Energy blaster perhaps the least of all.
Your 'iconic' hero sounds and looks similiar to the one I created way back in May of 2005.
Check the sig. It's my main.
Edit: Forgot I changed his main pic to have his level 50 costume. The battle pic has the outfit he started with.
Blue and White (designed before I knew the colors of energy blast) with the Peak pattern and a hood. Kinda hard to tell though.
Maestro Mavius - Infinity
Capt. Biohazrd - PCSAR
Talsor Tech - Talsorian Guard
Keep Calm & Chive On!
I agree that Energy/Energy Blasters are probably still considered the iconic combo.
Back when City of Heroes was sometimes derisively called "City of Blasters," and long before Power Proliferation, Energy/Energy Blasters were in fairly high demand because many of their powers were simply more effective than those of other Blasters. Fire develops later, Assault Rifle wasn't as powerful, Electricity had a bug while Ice has always been more about control than damage output. So a lot of people saw the sheer damage Energy impressively put out at the lower levels and so they chose that over others.
The only other real contender for "most used powerset" back then was Regen Scrapper, because Regen was far, far more powerful than it is today. (In Founders Falls in the early days, I once watched a Spines/Regen Scrapper take on a good twenty 5th Column that were 4-5 levels higher than he was, solo, and he walked away. That day I knew Regen was getting nerfed soon.) But the difference between Regen and NRG is that NRG is flashy and loud -- so not only was the AT more capable of mowing down enemies, it did so while calling attention to itself.
I had been a Defender player in CoH's early days, but my SG was lousy with Defenders at the time, so I decided to roll up a Blaster. I chose Energy purely because I liked how the powers looked. But very soon that character was the workhorse of the SG, racking up more kills than anyone else, essentially relegating all of my friends' characters to support roles. I based his costume on the color of the powers.
The Alt Alphabet ~ OPC: Other People's Characters ~ Terrific Screenshots of Cool ~ Superhero Fiction
I believe that something /Rad is more iconic. Radiation Emission is one of those uniquely CoX powersets that you just won't find anywhere else. It's also very flashy and lends itself to themes well.
Runners up would be Storm Summoning and Earth Control, simply for how visual the sets are and how unique they are to CoX. I would say Kinetics, but it's not visual enough to represent itself well in screenshots or pictures.
http://www.virtueverse.net/wiki/Massacre_Melanie -the original Fire/Dark Corruptor -
http://boards.cityofheroes.com/showthread.php?t=115217
The Guide to BURN
Is this something related just to CoH, or to Comic book superheroes in general? I ask because although i think energy/energy blasters certainly are an iconic superhero theme, i also think the super strength/invulnerability set is up there too, and would prolly prompt a totally different kind of costume. Food for thought 
Just CoH related
Volt Sentinel Reference
Thunder is good, thunder is impressive, but lightning does the work.
I'm putting this here rather than in the costume help thread cause it's a general question on everyone opinion.
I recently gather that most of the forum community consideres the Iconic powers (the powers that first come to mind when thinking about CoH) Seem to be an NRG/NRG Blaster..
I'm coming to the second part of the question. If you were to design a hero based on this "Iconic CoH character" what would you create?
Mine would be a blue/white Tights outfit, the powers would match the costume. pattern would be pattern 1. (i'd post a picture but I'm still trying to get Windows 7 to do that)
edit: or, if you think the Iconic CoH power is something other than NRG/NRG, what do you think it is?
Volt Sentinel Reference
Thunder is good, thunder is impressive, but lightning does the work.