I guess I don't understand what you mean by, "it got added later." Allison asking Ian if he would like her blonde, on a love route, is the most recent Ian update and was in the alpha of that update. There was not a way of missing it, unless there was a coding mistake I'm unaware of. If she is involved in any way with Jeremy, or if Ian treats her as a fuck buddy, she will go blonde without Ian's input. He can comment on it, but she won't (i.e. can't) change it back. That was, as far as I can remember, also in the alpha of when it was first implemented.
Are you saying that you haven't played the recent updates then?
She kind of already does? A) She's asked Ian if he would like her to dye her hair. B) Her wardrobe has probably changed the most throughout the game just short of Cindy. I'm not sure what else she could realistically change on a loving route at this point. Another wardrobe update would be most likely. The return of her belly ring perhaps? Although that's probably being reserved for her bimbo route if it does return.