How do you chat with someone who isn't a friend, after blocking and then unblocking them?

[Ranjana-star]

On facebook, I used to be able to chat with this person who wasn't a friend before but then after blocking and then unblocking them, I can't send them a message.

 

[learningbitbybit]

It could be because they set their privacy settings to allow only friends to send them private messages.