According to Experts: Xena, the character of Xena could have been inspired by several characters from Greek mythology.
However, I would like to offer another source of inspiration. Xena is described as someone, who was evil in the past, but repented and is now virtuous. The hypothetical source of inspiration, who was in vogue about 30 years earlier, was described as evil. Both of them had coal-black hair and they both were sexy and domineering women.
Each of them even has a female companion, with blond hair and very youthful look. There is also an hint of Lesbian relationship in both pairs.