It used to be that when the night of the Tebilah (ritual immersion), fell on Tish'ah BeAb, that it was permitted to be done then. However, nowadays we do not do so. The preparations of the bathing and washing of the hair, on the other hand, are done before the start of Tish'ah BeAb, and after Tish'ah BeAb the woman should wash her hair lightly, since this is supposed to be done just before immersing.
It goes without saying that if the night of the Tebilah falls on Mosei Tish'ah BeAb, that these preparations are forbidden on Tish'ah BeAb itself. In this case also, she should bathe and wash her hair before Tish'ah BeAb and then wash her again lightly, after the fast, before going to the Miqweh.
If, for some reason, she did not do the preparations before Tish'ah BeAb, she is permitted to do them after the fast, after the fact. However, this is not the correct way to do it.
(See Shulhan 'Arukh, 554:8. Kaf Hahayyim, ibid., Oth 41-43)
9th of Ab, Tish'ah BeAb, Miqweh, Mikvah