Anxious blog owners reported that full size images are now being displayed in a normal browser window, with the browser back arrow required to return to the blog display - instead of the elegant black background overlay with the mysterious "X" which closes the overlay. The problem appears to affect all images in the blogs affected.
An immediate sampling of the forum problem reports suggested that it's likely that all Blogger blogs are affected. Since not all blogs may contain enough images, and be owned by people who are attentive enough, we are not seeing massive volumes of problem reports.
We did open a rollup discussion, in Blogger Help Forum: Something Is Broken, where we are gathering a good sample of the blogs involved. To date, no obvious affinity has been discovered, in the over 100 responses posted.
Today, we have a mysterious suggestion, posted in a forum discussion, which may provide a workaround for the problem.
We are advised to add a brief snippet of code to the template.
The snippet in question is added to the very end of the template, using the "Edit HTML" option, in the dashboard Template wizard, after </body>, and before </html>.
I added this snippet of code to the template for this blog. I don't use a lot of images in this blog - but you may examine one previous post, where you will find a picture. If you click on that one picture, you will see the full size image displayed in Lightbox format - which was not the case, earlier today.
The code snippet in question is hosted in "blogger.com", so it should be safe. And, it's not difficult to add - or to remove. That said, I have no idea if it may conflict with other code - so anything is possible. It is marginally more reliable than third party code.
I've done no further testing of this fix, so I'll await your feedback. You may comment here - or in the rollup discussion.