Just to clarify, the code button does 2 things: if the selected text is one word or less than one line (no line breaks), it will create an inline code, otherwise a codeblock (starting with 3 backticks, line break, code, line break, and 3 backticks) that spans several lines.
Your request is feasible. Personally I've never much needed blockquotes, but I understand that it would make things easier for people who do.
I'm confident that Laurent would accept a PR for it. I can have a look at it, but I believe @CalebJohn wrote the code that distinguishes between inline code and code block. Maybe he has an idea how to implement this.
Another possible idea I would like to add is to just have a kind of “Other” or “More” or “Syntax Help” kind of button that opens some dialog or page that gives you the markdown guide for all of the needs (in addition to those most common ones covered by the buttons on the editor).
Today, for example (and the reason I am here typing this now), is because I had to look up how to do blockquotes through a google search for “GitHub Markdown”. It was easy enough, once I learned the syntax, but I too was starting by looking for the button in the editor.
Yes! Perfect! A link directly to the markdown syntax page would, in my opinion, be very helpful and also enough. While others may disagree, I do not think it would be necessary to muddy the editor bar with a ton of buttons as long as there was a quick way to jump to the syntax reference for looking up the less common things. The web page you provided the link for is perfect. It is well designed, easy to read and understand; it makes it very easy to quickly find what you are looking for. I know there is a link to the Website and Documentation under Help (or F1). Now that I have looked at it, I can see that it’s not hard to find the Markdown guide. To be honest, however, I didn’t know it was there or that easy to get to. I searched “GitHub Markdown” for my answer initially. Something closer to the editor bar or more explicit in the Help menu may be helpful to other users too.