Strange.
Only information I seem to be able to locate on this is pretty much what was previously provided. Shouldn't mean that the contact is blocked though (or can you provide a link to where you saw that ignore also means blocked, as in won't receive messages, etc)
What does the “ignore contact” button do on the new ios 11?
Which leads to What does the ignore contact feature do? (previously provided)
Pointing to the previously highlighted -
I had this issue. So I contacted apple who were unfamiliar of this therefore contacted the engineers. He confirmed..
"One of the features that came with iOS 10 is "Contacts Found in Apps" and when this option is enabled in "Settings > Contacts" you would receive unconfirmed contact suggestion that were found (e.g.) in emails and messages stored on the device. So the Ignore contact option just means that it will no longer suggest that specific contact to be added to your contacts. You can still receive all of their messages. If you want to add them to your contacts at a later time you can always create a new contact at that time and add them."
Hope this helps !
Though if still concerned that ignoring has meant that you will no longer receive messages, etc (effectively blocked them) then please Contact - Official Apple Support - Select iPhone -> iCloud, FaceTime, and Messages (since that it where you ignored the person), -> This topic is not listed -> Entered that you Ignored a contact that you wish to unignore
That should then allow you to get in touch with Apple directly.
If you do that, please then share the knowledge you have gained to help others!