Her father's kingdom was invaded by warriors of the Red Branch of Ulster, led by Cú Roí and Cú Chulainn. The raid led to her capture, along with several cattle and a magic cauldron. Despite her being in love with Cú Chulainn, she was chosen by Cú Roí as his personal spoil and she therefore married him, leading to a dispute between the two warriors. This ended with Cú Chulainn being shaved and humiliated by Cú Roí.
Later, she betrayed her husband to his enemies, pouring milk into the River Finglas (Finnglas) as a signal that he was at home. Subsequent to this action, Cú Roí was slain by Cú Chulainn. In revenge, Cú Roí's poet Ferchertne, threw both himself and Blathnát from a cliff.
Blathnat makes an appearance in the Fled Bricrenn (Feast of Bricriu) as the wife of the absent Cú Roí. She tells each of the three heroes to guard the fort on different nights, however only Cú Chulainn is able to defeat the force. Because of this, Cú Roí gives Cú Chulainn the Champion's Portion that he had been quarreling with the other heroes for, however the other two demand a rematch.