The common response for a Whitelighter is to retreat when seeing a Blacklighter, because Blacklighters tend to spend a significant portion of their time developing their fighting skills and quickly surpass the skill of most Whitelighters. Therefore, most Whitelighters have either the option to retreat, or be killed.
A few Blacklighters are friendly though, and will not harm you. Manny Many of them are happy to help new players that they encounter, especially since it gives them the opportunity to recruit to their side.