Hi Talk_Of_Town,
Thank you for participating in the Apple Support Communities.
We understand you've accidentally reported a message as junk and would like to undo it.
Check out the bottom of this article: Block phone numbers, contacts, and emails on your iPhone, iPad, or iPod touch
Tap Report Junk, then tap Delete and Report Junk. Messages will forward the sender's information and the message to Apple, as well as delete the message from your device. You can't undo deleting a message. Reporting junk doesn't block the sender from being able to send another message. If you don't want to receive these messages, you need to block the contact.
We’re recommending the steps above because they directly address the issue you initially posted about: you accidentally marked a message as junk.
Please do not hesitate to contact us if you have more questions.
Cheers!