Agree with moritz; this is a fundamentally bad idea, especially since there are occasions when one might wish to use nested quoting something like that below:

metaperl said "ww remarked 'That's a really bad idea, because, as paco observed (in a different context) that will create a "need to recurse" which will not serve us well.' But," metaperl continued, "On the other..."

In other words, you'll swap the existing need to type in <blockquote>...</blockquote> for a requirement to use character entities, &nbsp, as the enclosures when blockquoting is NOT desired.