Hi, Jslim26!
Thanks for stopping by the Apple Support Communities. From your post, I understand that you accidentally marked a message a junk in iMessage when it wasn't, and you're concerned that the number has been blocked so you won't receive other messages from this person. I'd be happy to help!
While marking a message as junk will send a report to Apple and delete the message from your device, it does not block the number. The sender will still be able to message and call you. You can get more information on that under the "Report spam or junk in the Messages app" section of Block phone numbers and contacts on your iPhone, iPad, or iPod touch - Apple Support. If you do want to block a number, you would do so following the steps outlined in the "Block a phone number or contact" section of the same article.
Have a wonderful day!