In earlier version(s) of the Visual Editor one could edit blockquotes. In later version(s) one can only edit them when they have just been inserted in the current edit session, but after saving the page and reopening it in the Visual Editor blockquotes are protected and cannot only be edited in Wikitext mode. I consider this a considerable regression.
Topic on VisualEditor/Feedback
Can you provide a link to a specific page where you experience this, and not which browser you are using ?
For instance, on this sandbox page: https://nl.wikipedia.org/wiki/Gebruiker:Coradriaan/Sandbox