I will host a blog using Github Pages, is it possible to add a secondary license in the assets folder so if people fork the project they would have to provide their own assets and not use the defaults one that came with the project?
I was thinking of a Creative Common Type of License for the assets folder in specific.
Yes. You can put files with any license you want in GitHub. However, note that one of GitHub's terms it that you grant GitHub enough rights to allow others to fork your code using the Fork button. So you won't be able to prevent others from mirroring your content on GitHub (though you may be able to pursue people who mirror your content outside of GitHub, depending on how (non)permissive your license is and other juicy aspects of copyright law).
