As of May 13, 2020, this bug has been fixed by Blackboard
Faculty attempting to use the default Blackboard YouTube Mashup tool

will be greeted with the following failure message …

Error: An error was returned by YouTube: There was a service error 403: Access Not Configured. YouTube Data API has not been used in project 28589479198 before or it is disabled.
Enable it by visiting https://console.developers.google.com/apis/api/youtube.googleapis.com/overview7projects28589479198 then retry. If you enabled this API recently, wait a few minutes for the action to propagate to our systems and retry.
You can safely ignore this error message. There is nothing that you can do to fix this feature. This error is generated because of a change that Google has imposed on this Blackboard Building Block.
We will repair this module as time affords. In the meantime, if you need to add a YouTube video to your Blackboard course then use the Kaltura Media Mashup Tool which is located at the bottom of the Build Content > Mashups > Kaltura Media Mashup tool.

This provides a more comprehensive approach to managing YouTube videos. The video integrated in this way becomes part of your larger Kaltura My Media collection. This allows your YouTube video to be housed in a collection that is not restricted to this one course.
According to Blackboard…
Recently Google decreased the daily quota limit for Learn so clients using the default YouTube API key are seeing 403 errors when users are searching for YouTube videos. We are working with Google to remediate…