Is blood discharged after intercourse on the seventh day of menstruation considered menstrual blood, and is expiation obligatory for the wife who did not inform her husband of that?

This returning blood is menstruation, because it occurred at a time suitable for menstruation. The intercourse that took place during the period of purity interspersed within the menstrual cycle was valid purity, so there is no sin upon either of you, nor any expiation, and you are not required to inform your husband.