Could it be acceptable to wear a kittle for 9 Av? I've never seen it, I usually wear black or dark grey. I wouldn't dare try to look innocent- I am guilty and I know it, but that doesn't stop me from wearing white on Yom Kippur. YK is about atonement, can 9 Av be about atonement also?
אבל בט' באב עצמו לא תלבש לבנים
But Tisha B'av itself, she should not wear whites rather wear a garment that is checked and nice [Haagos Shaarei Dora].
One wears a Kittel on Yom Kipper (and at the Seder on Pesach) either because
- It is due to Simcha (happiness); and one is being like a Malachei Hasharess (ministering angel). Or
- So that one remains humble; one wears a garment that is meant for the deceased. (Igros Moshe O.C. 5:61:7 quoting Remah O.C. 610:14)
According to #2, it would be appropriate to wear a Kittel on Tisha Baav. But, as you write, no one seems to do it.