I wouldn't worry about overpowered vs. underpowered, though it is something that always must be considered when adding a new house rule. I'd worry about if it is attractive to players.
I think it is a mistake to make any racial power an at-will; it only works for humans because it fits with the entire 'excelling at what you already do best'. Making something an at-will could be either next to useless for regular characters or extremely powerful for that player that sees a possibility for abuse.
Most races abilities are not specifically good depending on the class. They help each class by doing something general (eladrin; teleportation is useful movement, elves; better chance to hit, halflings; chance of not being hit, warforged; temporary hitpoints, tieflings; extra general damage). If they fit a certain role (say, and elf striker) it is nice, but it can also support a weakness (say, a warforged controller with few HP). No matter what, the racial benefit is useful.
With eldritch blast as a racial power, what happens when that person chooses a wizard? Their strength and constitution could be quite low (meaning the attack isn't very powerful), and they could choose something strictly better (magic missile, since their int will be high). Some might say "then they would choose a race better suited, or more useful". True, unless that particular person really loved the fluff you had about that race (I currently have a character that wants to be a Kalashtar Swordmage...). Even if something can be strictly better (say, a Dragonborn wizard/sorcerer who has a close blast 3 power), it is still useful (the dragonborn's racial power is a minor action, so it actually enhances the damage the player can put out on that one turn in the close blast 3 area).
Sorry for the long tangent. I'll say this one last time though: make a cool encounter power (perhaps with a similar feel as eldritch blast), not just rip a boring at-will power!