Is this plugin a blackhat? http://wordpress.org/extend/plugins/wp-noexternallinks/ I don't want google to give me bad ranking if this plugin is blackhat
That's not kinda blackhat plugin or something. This plugin will (only) change the structure of your link--add it with nofollow relation. Which means: the link(s) with this rel SHOULD NOT be afforded any additional weight or ranking by user agents which perform link analysis upon web pages (e.g. search engines). I'ts usually used by SPAMMERS to get a lovely backlink from your sites/blogs. Without nofollow rel, your sites/blogs will give external links for them (or counted by Google as a backlink), and it's very bad for your PageRank. cmiiw