Sally as it exists currently is extremely lackluster for its cost. It is usually too frail to make much of an impact, and its high operation cost makes it a serious investment to use.
Whenever I have Sally in hand, I usually find it better used as discard fodder than an actual bomber. The only time I consider using it is only if I’m desperate or if I already control the frontline.
Every other Japanese bomber is able to immediately influence the board, has an effect that can be exploited on the same turn it’s deployed, or has a method of evasion to avoid being attacked. Sally has none of these.
Giving it blitz lets it actually do something while on the disadvantage that isn’t dying immediately on deployment. Being able to threaten an instant 5 damage is pretty threatening.
I don’t think this would make it overpowered. You’ll also need at least 7k to use it on the same turn it’s deployed, and it’s gonna trade with fighters at best.
tl;dr: Sally’s lack of protection or immediate effect makes it practically useless unless you’re already winning. This buff gives it a way to contribute without copious amounts of protection.