Here's a simple regex solution I came up with. It's off the cuff and has a few quirks, but it might give you some ideas:

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>
[download]

And the output is:

<p><b>
blah blah
</b><blockquote><b>
blah
</b></blockquote><b>
blah
</b><p><b>
</b>
[download]

You'll note the fact that the <b>'s get inserted before and after the newlines in a strange fashion. That's one of the quirks. Another one is the <b> at the end that encloses nothing except a newline. This is because [^>] matches any character, including a newline character. You can tweak to suit if this is a problem.