Google has added one line to its video best practices document explaining that you should have at least 80% of your thumbnail pixels have little or no transparency to enable video indexing.
The new line was added over here and says “Transparency: At least 80% of the thumbnail’s pixels must have an alpha (transparency) value greater than 250.”
This is specifically under the “ensure your videos can be indexed” section of the document.
So if you want those videos or video thumbnails indexed, make sure they are not that transparent.
Forum discussion at Twitter.