actually your RewriteRule says to redirect to /article1.html/ on the current hostname, so i would expect it to redirect to http://blog.example.com/article1.html/ rather than http://example.com/article1.html
i would try something more like this:
RewriteRule ^article1/$ http://example.com/article1.html? [R=301,L]
you don't need the NE flag here and you don't want the NC flag.