How can I use a different template for the blog site I want to use k2 for and keep the template I am using on an existing site?

This is Joomla functionality. Set your default template - Then click on the other template, scroll down and click select and select the menu itme you want to use the different template for. give it a test!
Also, K2 is designed to allow K2 subtemplates that affect only the content area that you see in the site.
read about it here
simon.getk2.org/component/k2/item/91-templating
and here
community.getk2.org/notes/Templating_in_K2