Across the network blockquotes are by far more commonly used than spoilers, so it seems unfortunate to sacrifice the readability/identifiability of blockquotes to differentiate them better from spoilers.
(This change would seem like a good idea if the usage of quotes vs. spoilers was something like 1:50, but across the network it's probably more like 50000:1.)
I totally understand the idea of making them visually distinct, but would suggest reverting the change for blockquotes and adapting the design of the spoilers instead.
(For example, by using a slightly different shade of yellow, or a subtle texture/pattern for the yellow in spoilers that "covers" the spoiler and disappears on reveal.)