local $_ = do { local $/ = undef; <DATA> };

s[ (?<=>)  # start at an '>'
   ([^<]+) # match all the following non '<' chars
 ]
 [<b>$1</b>]gx;

print;

__DATA__
<p>
blah blah
<blockquote>
blah
</blockquote>
blah
<p>