First, what version are you looking at. In Telligent Community 5.0, inside the communityserver.config, there's a CSModule called "WeblogPostandArticleHtmlScrubbing" - you can comment out or remove that module and you can then add <script> tags to blog posts (using the HTML view). Please note, however, that in Telligent Community 5.0, blogs are a little less controlled than in other versions because blogs are contained within groups and it's easier to have regular system users as bloggers.
I thought there was another module that handled blog comment scrubbing and this may be handled by the ForumHtmlScrubbing module but there are no rich text editors in blog comments that allow a user to go into HTML to create script tags so all text is encoded to text (you can't create a script tag, it becomes <script>, etc).
I ran into a problem playing with this, however - when I commented out the module, I added <script>alert('Hi!');</script> but it got rendered in a way that was visible in source but wouldn't execute. That might be something caused by the editor or nesting correction or something - I'm not sure.
Hopefully this took care of your questions, though! Let me know if you could use clarification on any of this!