You may be referring to the shirt I've got. It's not royal blue, it's Davis Square sign blue, which has more white in it that royal blue - it's somewhere on the continuum between royal blue and sky blue. Also it doesn't exactly have the sign, because the real sign has both street names and a white outline and this has neither; on the other hand, it's not purely one-color since it does have a couple of red do not enter logs (though I wouldn't mind a similar shirt lacking those). However it might still be that that's what you meant.

I don't remember what year it was and what else, if anything, this guy was selling, but I'll see if they can figure it out from such vague information :) Although it sounds like, if that's true, whoever it was likely either isn't around anymore or has no more shirts to sell or probably both.