This is intended and there is no way to revert. For more information and discussion about the matter, see here.
A quick summary of the motivation, found in the aforementioned post:
We’re making a few changes to blockquotes soon. Historically, we had a bit of feedback that the yellow backgrounds were overpowering, reducing the contrast of the quote itself. The yellow border also lacks contrast with the background. Others have indicated that there should be more visible differences between block quotes and spoilers and between code and quotes.
These yellows also compete with theming in our communities, especially those with backgrounds that aren’t perfectly white like English Language & Usage and Role-Playing Games.
We’re proposing we switch to a single gray bar in our blockquotes. This should fix those theming issues and make it easier to distinguish between quotes, spoilers and code.
This will also improve blockquote nesting, since our current nesting is a mess because the darker yellow bar is barely noticeable on the paler yellow background.