I recently had to get a new PC for my Blue Iris installation to reside on.

I installed Blue Iris, Imported my settings, then attempted to activate with my key. As soon as I hit "Activate", the watermark goes away...until I close the settings dialog, then it comes back and persists.

My question is this, do I need to deactivate the key from the old machine whilst connected to the internet?

Yes.

Also if you reinstalled then you will be on the latest release I suspect so do you have active support? If not then you need to go back to a version that was released on or before the expiry date of your support.

Well, in a unfortunate series of events, the HDD that old installation was on is no longer viable. So I have no access to the original installation.

BUT! I found an email from earlier this year with an automated purchase of the Extended Maintenance and Support. It included a new license key to be added in the "upgrade/renew" section.

This has solved my issue!

Thank you for your time!