<blockquote> is in the "Paragraph" (not character formatting) menu in the visual editor. It can be reached by the keyboard shortcut Ctrl-8. When you do this, the menu still displays "Paragraph" (Ctrl-0).

What should happen:

• Place cursor in a paragraph.
• Type Ctrl-8 to make it a blockquote. See "Block quote" in the Paragraph menu.
• Decide that I don't want to have it be a block quote after all.
• Type Ctrl-0 to make it a normal paragraph. See "Paragraph" in the Paragraph menu.

What actually happens:

• Place cursor in a paragraph.
• Type Ctrl-8 to make it a blockquote. See "Paragraph" in the Paragraph menu.
• Decide that I don't want to have it be a block quote after all.
• Type Ctrl-0 to make it a normal paragraph. Nothing happens. The text is still a block quote.

You can remove block quote formatting either by typing Ctrl-8 (because it's a toggle, like bold) or by changing it to some third option in the paragraph menu (e.g., typing Ctrl-2 to make it a ==Section heading==), and from there to switch it to Ctrl-0/plain old paragraph.

This also happens in the Reply tool.

This is probably related to T221692: Consider moving the blockquote tool to another section of the toolbar.