Jewbacca, the problem with using a negative lookbehind as in your suggestions (apart from the syntax in your first one - that is a negative lookahead that uses the string that starts with an equals sign and colon) is that, on its own, it doesn't solve the problem.
Take the word "blogshpere". When the pattern starts on that word, there is nothing to look behind at and so the rest of the pattern matches quite happily.
What you need to do is to anchor the pattern to force it to start at the beginning of the word. Once you have that anchor, you can then use a negative lookahead to check for the "blog" character string and go from there. My suggestion would be:
\b(?!blog)[a-z0-9]+
of course using the 'ignore case' and whatever other options and adding matching groups etc as needed.
Susan