For some reference as to how block quotes used to look like on EL&U before the the theme was overhauled, see English Language & Usage's updated site theme is ready for testing! and Make blockquotes inside blockquotes better distinguishable .
Since December 2018
According to a Meta announcement dated February 19, 2020, this will soon change.
Some improvements to blockquotes
We’re making a few changes to blockquotes soon. […]
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.
An example of how the proposed change will look:
(Yes, I could've cropped it better)
Apparently the grey sidebar is used on similar sites
The text says: There is a lot of prior art out there for this approach. GitHub has styled theirs this way for a long time. So has Slack
On EL&U, which has a yellow theme, it should look something like this
The above is a cropped image taken from a much larger one found here.
The Stack Developer/employee, Aaron Shekey♦ concludes with this message.
Let me know what y’all think. I think these changes will be an improvement, and I hope you do too.
Well, seeing as many users on EL&U are probably unaware of the proposal, because I haven't seen any announcement on EL&U meta, please make yourself heard on META (Stack Exchange) and post what y'all think.