On Blog Comments editor [Blockquote] is already enabled. There is an Idea asking for improved editor... although there are some concerns about it as well. You could add your voice there and yes, I think the [Blockquote] could come to Q&A (and Coffee Corner) as well.
you can click on CODE and it will give you box like below to paste your quote.
Hi guys, How do I quote text in the new editor? Thanks.
Thanks
Ritesh