this is a feature that would have to be added on Mastodon's end, since it has to do with how they're handling incoming HTML tags
Markdown is not used for communication between servers. Mastodon does not have access to any of the original Discourse markdown that you type in, it can only see the rendered HTML tags. However, I agree that adding some sort of indicator for block quotes would probably be good—its just harder than "show the original markdown" makes it seem