Here to show my support for blockquote as well! I was quite surprised to see it wasn't available in rich text fields yet.
I'd rather not create workarounds for it.
If I'm not mistaken, the html <code> tag is also missing. I suppose <pre> could be used instead.
Some other useful tags I'd love to see:
- <abbr>
- <details> + <summary>
- <table> and its associated tags (the current workaround seems like a less than ideal editing experience.)
If you're open to suggestions regarding <table>, the authoring experience on HubSpot could be a great inspiration: Insert a table in a rich text module