Question about Private Messaging

I sent a private message to someone that I'm not friends with on FB. I immediately regretted it. It did not show that she had viewed it...in fact the little circle with a check mark was not even filled in with blue. The only way possible that I thought I could keep the message from going through was to block her. So, my question is, if you send a private message to a person that you're not friends with and you block that person before they have viewed the message, will they still be able to see the message since they had not yet viewed it. I'm nervous and would appreciate if someone might have a solid answer regarding this. Thanks in advance!!