A primary blog is the one you created when you signed up for Tumblr. It gives you full use of Tumblr’s social features, including follow, like, ask, and submit.
Secondary blogs are any blogs you create in addition to your primary blog. You can create up to 10 of these per day. Secondary blogs can also:
- Be password-protected, if you like
- Have multiple users who can create and manage posts
No switching or combining blogs, but you can move a URL
Unfortunately, you can't turn one of your secondary blogs into your primary blog. If you want to move a URL from one blog to another, you can do so by (1) changing the URL of the existing blog to something new so the URL is freed, then (2) changing the URL of a different blog to the freed URL, or creating an entirely new account with the freed URL.