Is it unhealthy for somebody to be a night owl if they still get an undistirbed 8 hours of sleep once a day with a sleep mask or in a dark room?

You can, BUT: With most humans, the natural circadian rhythm is to be awake in daytime and asleep at night. Unless you are very careful to get enough good-quality sleep, you can end up sleep deprived with all ill effects from that. Some people are natural "night owls" though. Many of those choose to work at night, using their reversed sleep-wake cycles to advantage.

No necessesarily: I you get a fully functional sleep and wake up refreshed there shouldn't be a problem.